Title of Invention: Anhydrous solid composition for dyeing keratin fibres comprising a metabisulfite Abstract: The present invention relates to an anhydrous solid composition for dyeing keratin fibres, in particular human keratin fibres such as the hair, comprising one or more metabisulfites. The present invention also relates to a packaging article containing said composition. The present invention also relates to processes for dyeing keratin fibres using said composition or said packaging article. Finally, the present invention also relates to the use of said composition or of said packaging article for dyeing keratin fibres, and in particular human keratin fibres such as the hair. Detailed Description: Part general_description_paragraphs: Part CROSS REFERENCE TO RELATED APPLICATIONS: This application is a National Phase filing under 35 U.S.C. § 371 of PCT/EP2017/083441 filed on Dec. 19, 2017; which application in turn claims priority to Application No. 1662927 filed in France on Dec. 20, 2016. The entire contents of each application are hereby incorporated by reference. The present invention relates to an anhydrous solid composition for the oxidation dyeing of keratin fibres, in particular human keratin fibres such as the hair, comprising one or more metabisulfites. The present invention also relates to a packaging article containing said composition. The present invention also relates to processes for dyeing keratin fibres using said composition or said packaging article. Finally, the present invention also relates to the use of said composition or of said packaging article for dyeing keratin fibres, and in particular human keratin fibres such as the hair. For a long time, hair dyeing, and in particular the masking of grey hair, has been desired by numerous individuals. It is known practice to dye keratin fibres, in particular human keratin fibres such as the hair, to obtain “permanent” colourations with dyeing compositions containing oxidation dye precursors, which are generally known as oxidation bases, such as ortho- or para-phenylenediamines, ortho- or para-aminophenols, or heterocyclic compounds such as pyrazoles, pyrazolinones or pyrazolo-pyridines. These oxidation bases are colourless or weakly coloured compounds, which, when combined with oxidizing products, may give rise to coloured compounds via a process of oxidative condensation. It is also possible to vary the shades obtained with these oxidation bases by combining them with couplers or colour modifiers. The variety of molecules used as oxidation bases and couplers allows a wide range of colours to be obtained on the keratin fibres. The oxidation dyeing process thus consists in applying to keratin fibres a dyeing composition comprising oxidation bases or a mixture of oxidation bases and couplers with hydrogen peroxide (H2O2 or aqueous hydrogen peroxide solution), as oxidizing agent, in leaving it to diffuse, and then in rinsing the fibres thus treated. The colourings resulting therefrom have the advantage of being permanent, strong and resistant to external agents, in particular to light, bad weather, washing, perspiration and rubbing. Although dyeing compositions can be in various galenic forms, such as powders, granules, pastes or creams, they are generally packaged in liquid form. However, solid dyeing compositions bring many advantages compared with liquid dyeing compositions. The process for producing them does not require water, thereby making it possible to adopt a more eco-friendly behaviour. It can also be miniaturized. However, the solid dyeing compositions currently on the market exhibit, after several weeks of storage, problems of stability which result in a premature oxidation of the dyes and a decrease in dyeing performance qualities once the composition is used. Moreover, once oxidized, the composition has a tendency to turn dark, which is not popular with consumers. In order to compensate for this oxidation, many dyeing compositions in solid form comprise amounts of oxidation dyes that are higher than the amounts of dyes contained in liquid compositions. Solving this oxidation problem by increasing the amounts of base of the dyes in the composition is not satisfactory. Indeed, this solution is not economically advantageous. Moreover, the handling and/or application of large amounts of dyes on the hair can pose a problem due to their toxicological profile. At the current time, there is no dyeing composition in solid form that is satisfactory in terms of stability and that has a degree of dyeing quality similar to liquid dyeing compositions. Finally, the formulations in solid form make it possible to use raw materials that are unstable or have low stability in liquid formulation. Thus, there is a real need to provide solid dyeing compositions which do not have the abovementioned drawbacks, i.e. which remain stable over time and which retain their dyeing properties. These compositions must also lead to chromatic, powerful, intense and sparingly selective colourings, that is to say colourings that are uniform along the length of the keratin fibre. The applicant has discovered, surprisingly, that an anhydrous solid dyeing composition comprising one or more oxidation bases and one or more metabisulfites makes it possible to achieve the objectives set out above, in particular to obtain a solid dyeing composition that is stable over time, and the dyeing properties of which are maintained even after several months of storage. A subject of the present invention is in particular an anhydrous solid composition for dyeing keratin fibres, in particular human keratin fibres such as the hair, comprising: one or more oxidation bases, one or more metabisulfites, and optionally one or more oxidation couplers. The present invention also relates to a ready-to-use composition comprising an anhydrous solid dyeing composition, as defined above, one or more chemical oxidizing agents, and optionally a cosmetically acceptable medium. The presence of metabisulfite(s) makes it possible to limit the oxidation of the composition, thus improving its stability over time. The composition thus stabilized retains its dyeing properties. In other words, the metabisulfite makes it possible to maintain the dyeing properties of the composition over time without recourse to an increase in the dye content. Thus, the composition according to the invention makes it possible to produce chromatic, powerful, intense and sparingly selective colourings, even after several months of storage. A subject of the present invention is also a packaging article comprising: i) an envelope defining at least one cavity, the envelope comprising water-soluble and/or liposoluble fibres, preferably water-soluble fibres; and ii) an anhydrous solid dyeing composition as defined previously; it being understood that the anhydrous solid dyeing composition is in one of the cavities defined by the envelope i). The present invention also relates to a process for dyeing keratin fibres, in particular human keratin fibres such as the hair, comprising the following successive steps: applying to said keratin fibres a ready-to-use composition as defined previously, leaving the ready-to-use composition on said keratin fibres, rinsing said keratin fibres, and optionally shampooing said keratin fibres, rinsing them and drying them. A subject of the present invention is also a process for dyeing keratin fibres, in particular human keratin fibres such as the hair, comprising the following successive steps: mixing a packaging article, as defined previously, with a composition capable of dissolving the envelope of said packaging article, and optionally one or more chemical oxidizing agents, applying the resulting composition to said keratin fibres, leaving said resulting composition on said keratin fibres, rinsing said keratin fibres, and optionally shampooing said keratin fibres, rinsing them and drying them. The present invention also relates to the use of a ready-to-use composition, as defined previously, for dyeing keratin fibres, and in particular human keratin fibres such as the hair. The present invention also relates to the use of a packaging article, as defined previously, for dyeing keratin fibres, and in particular human keratin fibres such as the hair. Other subjects, characteristics, aspects and advantages of the invention will emerge even more clearly on reading the description and the examples that follow. In the text hereinbelow, and unless otherwise indicated, the limits of a range of values are included within that range, especially in the expressions “between” and “ranging from . . . to . . . ”. Moreover, the expressions “at least one” and “at least” used in the present description are equivalent to the expressions “one or more” and “greater than or equal to”, respectively. The term “keratin materials” preferably denotes human keratin materials, such as the skin, the scalp and keratin fibres that are human, and more preferentially the hair. The term “anhydrous composition” is intended to mean a composition comprising a water content of less than 3% by weight, preferably less than 1% by weight, relative to the weight of the composition. Preferably, this water content is less than 0.5% by weight relative to the weight of the composition. More particularly, the water content ranges from 0 to 1% by weight and preferably from 0 to 0.5% by weight relative to the total weight of the composition. Finally, more particularly, it does not comprise water. The term “solid composition” is intended to mean a composition that can be in powder, paste or particle form (for example spherical particles such as small balls). The term “powder” is intended to mean a composition in pulverulent form, which is preferably essentially free of dust (or fine particles). In other words, the particle size distribution of the particles is such that the weight ratio of particles less than or equal to 100 micrometres in size (fines content) and preferably less than or equal to 65 micrometres in size (fines content) is advantageously less than or equal to 5%, preferably less than 2% and more particularly less than 1% (particle size evaluated using a Retsch AS 200 Digit particle size analyser; oscillation height: 1.25 mm/screening time: 5 minutes). Advantageously, the particle size is between 100 μm and 3 mm and more particularly between 65 μm and 2 mm. The term “paste” is intended to mean a composition with a viscosity of greater than 5 poises and preferably greater than 10 poises, measured at 25° C. and at a shear rate of 1 s−1; this viscosity may be determined using a cone-plate rheometer. The term “particles” is intended to mean small fractionated objects formed from solid particles aggregated together, of variable shapes and sizes. They may be regular or irregular in shape. They may in particular have a spherical shape (such as granules, granular material, balls), a square shape, a rectangular shape, or an elongated shape such as rods. Spherical particles are quite particularly preferred. The size of the particles can be, in the largest dimension thereof, between 0.01 and 5 mm, preferably between 0.1 and 2.5 mm, and better still between 0.5 and 2 mm. The anhydrous solid composition according to the invention can be in the form of a compressed solid composition, in particular compressed using a manual or mechanical press. Anhydrous Solid Dyeing Composition Oxidation Bases The anhydrous solid dyeing composition according to the present invention comprises one or more oxidation bases. Preferably, the oxidation bases are chosen especially from heterocyclic bases and benzene-based bases, the addition salts thereof, the solvates thereof, and mixtures thereof. The oxidation bases that may be used in the composition of the invention are chosen especially from para-phenylenediamines, bis(phenyl)alkylenediamines, para-aminophenols, ortho-aminophenols and heterocyclic bases, the addition salts thereof, the solvates thereof, and mixtures thereof. Among the para-phenylenediamines that may be mentioned are, for example, para-phenylenediamine, para-toluenediamine, 2-chloro-para-phenylenediamine, 2,3-dimethyl-para-phenylenediamine, 2,6-dimethyl-para-phenylenediamine, 2,6-diethyl-para-phenylenediamine, 2,5-dimethyl-para-phenylenediamine, N,N-dimethyl-para-phenylenediamine, N,N-diethyl-para-phenylenediamine, N,N-dipropyl-para-phenylenediamine, 4-amino-N,N-diethyl-3-methylaniline, N,N-bis(β-hydroxyethyl)-para-phenylenediamine, 4-N,N-bis((3-hydroxyethyl)amino-2-methylaniline, 4-N,N-bis((3-hydroxyethyl)amino-2-chloroaniline, 2-β-hydroxyethyl-para-phenylenediamine, 2-methoxymethyl-para-phenylenediamine, 2-fluoro-para-phenylenediamine, 2-isopropyl-para-phenylenediamine, N-((3-hydroxypropyl)-para-phenylenediamine, 2-hydroxymethyl-para-phenylenediamine, N,N-dimethyl-3-methyl-para-phenylenediamine, N-ethyl-N-((3-hydroxyethyl)-para-phenylenediamine, N-(β,γ-dihydroxypropyl)-para-phenylenediamine, N-(4′-aminophenyl)-para-phenylenediamine, N-phenyl-para-phenylenediamine, 2-β-hydroxyethyloxy-para-phenylenediamine, 2-β-acetylaminoethyloxy-para-phenylenediamine, N-((3-methoxyethyl)-para-phenylenediamine, 4-aminophenylpyrrolidine, 2-thienyl-para-phenylenediamine, 2-β-hydroxyethylamino-5-aminotoluene and 3-hydroxy-1-(4′-aminophenyl)pyrrolidine, and the corresponding addition salts with an acid. Among the para-phenylenediamines mentioned above, para-phenylenediamine, para-toluenediamine, 2-isopropyl-para-phenylenediamine, 2-β-hydroxyethyl-para-phenylenediamine, 2-β-hydroxyethyloxy-para-phenylenediamine, 2,6-dimethyl-para-phenylenediamine, 2,6-diethyl-para-phenylenediamine, 2,3-dimethyl-para-phenylenediamine, N,N-bis((3-hydroxyethyl)-para-phenylenediamine, 2-chloro-para-phenylenediamine and 2-β-acetylamino ethyloxy-para-phenylenediamine, and the corresponding addition salts with an acid, are particularly preferred. Among the bis(phenyl)alkylenediamines that may be mentioned, for example, are N,N′-bis((3-hydroxyethyl)-N,N′-bis(4′-aminophenyl)-1,3-diaminopropanol, N,N′-bis((3-hydroxyethyl)-N,N′-bis(4′-aminophenyl)ethylenediamine, N,N′-bis(4-aminophenyl)tetramethylenediamine, N,N′-bis((3-hydroxyethyl)-N,N′-bis(4-aminophenyl)tetramethylenediamine, N,N′-bis(4-methylaminophenyl)tetramethylenediamine, N,N′-bis(ethyl)-N,N′-bis(4′-amino-3′-methylphenyl)ethylene diamine and 1,8-bis(2,5-diaminophenoxy)-3,6-dioxaoctane, and the corresponding addition salts. Among the para-aminophenols that are mentioned are, for example, para-aminophenol, 4-amino-3-methylphenol, 4-amino-3-fluorophenol, 4-amino-3-chlorophenol, 4-amino-3-hydroxymethylphenol, 4-amino-2-methylphenol, 4-amino-2-hydroxymethylphenol, 4-amino-2-methoxymethylphenol, 4-amino-2-aminomethylphenol, 4-amino-2-((3-hydroxyethylaminomethyl)phenol and 4-amino-2-fluorophenol, and the corresponding addition salts with an acid. Among the ortho-aminophenols that may be mentioned, for example, are 2-aminophenol, 2-amino-5-methylphenol, 2-amino-6-methylphenol and 5-acetamido-2-aminophenol, and the corresponding addition salts. Among the heterocyclic bases that may be mentioned, for example, are pyridine, pyrimidine and pyrazole derivatives. Among the pyridine derivatives that may be mentioned are the compounds described, for example, in patents GB 1 026 978 and GB 1 153 196, for example 2,5-diaminopyridine, 2-(4-methoxyphenyl)amino-3-aminopyridine and 3,4-diaminopyridine, and the corresponding addition salts. Other pyridine oxidation bases that are useful in the present invention are the 3-aminopyrazolo[1,5-a]pyridine oxidation bases or the corresponding addition salts described, for example, in patent application FR 2 801 308. Examples that may be mentioned include pyrazolo[1,5-a]pyrid-3-ylamine, 2-acetylaminopyrazolo[1,5-a]pyrid-3-ylamine, 2-(morpholin-4-yl)pyrazolo[1,5-a]pyrid-3-ylamine, 3-aminopyrazolo[1,5-a]pyridine-2-carboxylic acid, 2-methoxypyrazolo[1,5-a]pyrid-3-ylamine, (3-aminopyrazolo[1,5-a]pyrid-7-yl)methanol, 2-(3-aminopyrazolo[1,5-a]pyrid-5-yl)ethanol, 2-(3-aminopyrazolo[1,5-a]pyrid-7-yl) ethanol, (3-aminopyrazolo[1,5-a]pyrid-2-yl)methanol, 3,6-diaminopyrazolo[1,5-a]pyridine, 3,4-diaminopyrazolo[1,5-a]pyridine, pyrazolo[1,5-a]pyridine-3,7-diamine, 7-(morpholin-4-yl)pyrazolo[1,5-a]pyrid-3-ylamine, pyrazolo[1,5-a]pyridine-3,5-diamine, 5-(morpholin-4-yl)pyrazolo[1,5-a]pyrid-3-ylamine, 2-[(3-aminopyrazolo[1,5-a]pyrid-5-yl)(2-hydroxyethyl)amino]ethanol, 2-[(3-aminopyrazolo[1,5-a]pyrid-7-yl)(2-hydroxyethyl)amino]ethanol, 3-aminopyrazolo[1,5-a]pyridin-5-ol, 3-aminopyrazolo[1,5-a]pyridin-4-ol, 3-aminopyrazolo[1,5-a]pyridin-6-ol, 3-aminopyrazolo[1,5-a]pyridin-7-ol, 2-β-hydroxyethoxy-3-aminopyrazolo[1,5-a]pyridine and 2-(4-dimethylpiperazinium-1-yl)-3-aminopyrazolo[1,5-a]pyridine, and the corresponding addition salts. More particularly, the oxidation bases that are useful in the present invention are chosen from 3-aminopyrazolo[1,5-a]pyridines and are preferably substituted on carbon atom 2 with: a) a (di)(C1-C6)(alkyl)amino group, said alkyl group possibly being substituted with at least one hydroxyl, amino or imidazolium group; b) an optionally cationic 5- to 7-membered heterocycloalkyl group comprising from 1 to 3 heteroatoms, optionally substituted with one or more (C1-C6)alkyl groups such as a di(C1-C4)alkylpiperazinium group; or c) a (C1-C6)alkoxy group optionally substituted with one or more hydroxyl groups, such as a β-hydroxyalkoxy group, and the corresponding addition salts. Among the 3-aminopyrazolo[1,5-a]pyridine bases, it will in particular be preferred to use 2[(3-aminopyrazolo[1,5-a]pyridin-2-yl)oxy]ethanol, and/or 4-(3-aminopyrazolo[1,5-a]pyridin-2-yl)-1,1-dimethylpiperazin-1-ium chloride and/or the corresponding addition salts or solvates thereof. Among the pyrimidine derivatives that may be mentioned are the compounds described, for example, in patents DE 2359399; JP 88-169571; JP 05-63124; EP 0770375 or patent application WO 96/15765, such as 2,4,5,6-tetraaminopyrimidine, 4-hydroxy-2,5,6-triaminopyrimidine, 2-hydroxy-4,5,6-triaminopyrimidine, 2,4-dihydroxy-5,6-diaminopyrimidine, 2,5,6-triaminopyrimidine and the addition salts thereof and the tautomeric forms thereof, when a tautomeric equilibrium exists. Among the pyrazole derivatives that may be mentioned are the compounds described in patents DE 3843892 and DE 4133957 and patent applications WO 94/08969, WO 94/08970, FR-A-2 733 749 and DE 195 43 988, for instance 4,5-diamino-1-methylpyrazole, 4,5-diamino-1-((3-hydroxyethyl)pyrazole, 3,4-diaminopyrazole, 4,5-diamino-1-(4′-chlorobenzyl)pyrazole, 4,5-diamino-1,3-dimethylpyrazole, 4,5-diamino-3-methyl-1-phenylpyrazole, 4,5-diamino-1-methyl-3-phenylpyrazole, 4-amino-1,3-dimethyl-5-hydrazinopyrazole, 1-benzyl-4,5-diamino-3-methylpyrazole, 4,5-diamino-3-tert-butyl-1-methylpyrazole, 4,5-diamino-1-tert-butyl-3-methylpyrazole, 4,5-diamino-1-((3-hydroxyethyl)-3-methylpyrazole, 4,5-diamino-1-ethyl-3-methylpyrazole, 4,5-diamino-1-ethyl-3-(4′-methoxyphenyl)pyrazole, 4,5-diamino-1-ethyl-3-hydroxymethylpyrazole, 4,5-diamino-3-hydroxymethyl-1-methylpyrazole, 4,5-diamino-3-hydroxymethyl-1-isopropylpyrazole, 4,5-diamino-3-methyl-1-isopropylpyrazole, 4-amino-5-(2′-aminoethyl)amino-1,3-dimethylpyrazole, 3,4,5-triaminopyrazole, 1-methyl-3,4,5-triaminopyrazole, 3,5-diamino-1-methyl-4-methylaminopyrazole and 3,5-diamino-4-(β-hydroxyethyl)amino-1-methylpyrazole, and the corresponding addition salts. Use may also be made of 4,5-diamino-1-((3-methoxyethyl)pyrazole. A 4,5-diaminopyrazole will preferably be used and even more preferentially 4,5-diamino-1-((3-hydroxyethyl)pyrazole and/or a corresponding salt. The pyrazole derivatives that may also be mentioned include diamino-N,N-dihydropyrazolopyrazolones and in particular those described in patent application FR-A-2 886 136, such as the following compounds and the corresponding addition salts: 2,3-diamino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one, 2-amino-3-ethylamino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one, 2-amino-3-isopropylamino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one, 2-amino-3-(pyrrolidin-1-yl)-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one, 4,5-diamino-1,2-dimethyl-1,2-dihydropyrazol-3-one, 4,5-diamino-1,2-diethyl-1,2-dihydropyrazol-3-one, 4,5-diamino-1,2-bis(2-hydroxyethyl)-1,2-dihydropyrazol-3-one, 2-amino-3-(2-hydroxyethyl)amino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one, 2-amino-3-dimethylamino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one, 2,3-diamino-5,6,7,8-tetrahydro-1H,6H-pyridazino[1,2-a]pyrazol-1-one, 4-amino-1,2-diethyl-5-(pyrrolidin-1-yl)-1,2-dihydropyrazol-3-one, 4-amino-5-(3-dimethylaminopyrrolidin-1-yl)-1,2-diethyl-1,2-dihydropyrazol-3-one and 2,3-diamino-6-hydroxy-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one. Use will preferably be made of 2,3-diamino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one and/or a corresponding salt. Use will preferably be made of 4,5-diamino-1-((3-hydroxyethyl)pyrazole and/or 2,3-diamino-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazol-1-one and/or 2[(3-aminopyrazolo[1,5-a]pyridin-2-yl)oxy]ethanol and/or 4-(3-aminopyrazolo[1,5-a]pyridin-2-yl)-1,1-dimethylpiperazin-1-ium chloride and/or the corresponding salts or solvates thereof as heterocyclic bases. In general, the addition salts of oxidation bases that may be used in the composition according to the invention are chosen in particular from the addition salts with an acid such as the hydrochlorides, hydrobromides, sulfates, citrates, succinates, tartrates, lactates, tosylates, benzenesulfonates, phosphates and acetates. Moreover, the solvates of the oxidation bases more particularly represent the hydrates of said bases and/or the combination of said bases with a linear or branched C1 to C4 alcohol such as methanol, ethanol, isopropanol or n-propanol. Preferably, the solvates are hydrates. Preferably, the oxidation base(s) are chosen from para-phenylenediamines, bis(phenyl)alkylenediamines, para-aminophenols, bis-para-aminophenols, ortho-aminophenols and heterocyclic bases, the addition salts thereof, the solvates thereof, and mixtures thereof. More preferentially, the oxidation base(s) are chosen from para-phenylenediamines, para-aminophenols, the addition salts thereof, the solvates thereof, and mixtures thereof, and preferably from toluene-2,5-diamine, 1-hydroxy-4-aminobenzene, the addition salts thereof, the solvates thereof, and mixtures thereof. The total amount of oxidation base(s) present in the anhydrous solid dyeing composition according to the invention preferably ranges from 0.1% to 45% by weight, more preferentially from 0.5% 40% by weight, better still from 1% to 35% by weight and even better still from 5% to 35% by weight, relative to the total weight of the anhydrous solid dyeing composition. Metabisulfites The anhydrous solid dyeing composition according to the present invention also comprises one or more metabisulfites. The metabisulfite(s) according to the invention can preferably be chosen from alkali metal or alkaline-earth metal metabisulfites and mixtures thereof, more preferentially from alkali metal metabisulfites and mixtures thereof, and better still from sodium or potassium metabisulfites and mixtures thereof. Particularly preferably, the metabisulfite is sodium metabisulfite. The total amount of metabisulfite(s) present in the anhydrous solid dyeing composition according to the invention preferably ranges from 0.1% to 30% by weight, relative to the total weight of the anhydrous solid dyeing composition. More particularly, the total amount of metabisulfite(s) ranges from 0.2% to 20% by weight, more preferentially from 1% to 20% by weight and better still from 3% to 10% by weight, relative to the total weight of the anhydrous solid dyeing composition. More particularly, when the metabisulfite is sodium metabisulfite, the total amount of sodium metabisulfite present in the anhydrous solid dyeing composition according to the invention preferably ranges from 0.1% to 30% by weight, relative to the total weight of the anhydrous solid dyeing composition. More preferentially when the metabisulfite is sodium metabisulfite, the total amount of sodium metabisulfite present in the anhydrous solid dyeing composition according to the invention ranges from 0.2% to 20% by weight, better still from 1% to 20% by weight and still from 3% to 10% by weight, relative to the total weight of the anhydrous solid dyeing composition. Oxidation Couplers The anhydrous solid dyeing composition according to the present invention can optionally also comprise one or more oxidation couplers conventionally used for dyeing keratin fibres. Among these oxidation couplers, mention may be made in particular of meta-phenylenediamines, meta-aminophenols, meta-diphenols, naphthalene-based couplers and heterocyclic couplers, the addition salts thereof, the solvates thereof, and mixtures thereof. Examples that may be mentioned include 2-methyl-5-hydroxyethylaminophenol, 2,4-diaminophenoxyethanol, 1,3-dihydroxybenzene, 1,3-dihydroxy-2-methylbenzene, 4-chloro-1,3-dihydroxybenzene, 2,4-diamino-1-(β-hydroxyethyloxy)benzene, 2-amino-4-((3-hydroxyethylamino)-1-methoxybenzene, 1,3-diaminobenzene, 1,3-bis(2,4-diaminophenoxy)propane, 3-ureido aniline, 3-ureido-1-dimethylamino benzene, sesamol, 1-β-hydroxyethylamino-3,4-methylenedioxybenzene, α-naphthol, 2-methyl-1-naphthol, 6-hydroxyindole, 4-hydroxyindole, 4-hydroxy-N-methylindole, 2-amino-3-hydroxypyridine, 6-hydroxybenzomorpholine, 3,5-diamino-2,6-dimethoxypyridine, 1-N-(β-hydroxyethyl)amino-3,4-methylenedioxybenzene, 2,6-bis(β-hydroxyethylamino)toluene, 6-hydroxyindoline, 2,6-dihydroxy-4-methylpyridine, 1-H-3-methylpyrazole-5-one, 1-phenyl-3-methylpyrazole-5-one, 2,6-dimethylpyrazolo[1,5-b]-1,2,4-triazole, 2,6-dimethyl[3,2-c]-1,2,4-triazole and 6-methylpyrazolo[1,5-a]benzimidazole, 2-methyl-5-aminophenol, 5-N-((3-hydroxyethyl)amino-2-methylphenol, 3-aminophenol and 3-amino-2-chloro-6-methylphenol, the corresponding addition salts with an acid and the corresponding mixtures. Preferably, the oxidation coupler(s) are chosen from meta-phenylenediamines, meta-aminophenols, the addition salts thereof, the solvates thereof, and mixtures thereof, and preferably from 2-methyl-5-hydroxyethylaminophenol, 2,4-diaminophenoxyethanol, the addition salts thereof, the solvates thereof, and mixtures thereof. The addition salts of the oxidation couplers optionally present in the composition according to the invention are chosen especially from the addition salts with an acid, such as the hydrochlorides, hydrobromides, sulfates, citrates, succinates, tartrates, lactates, tosylates, benzenesulfonates, phosphates and acetates, and the addition salts with a base such as sodium hydroxide, potassium hydroxide, ammonia, amines or alkanolamines. Moreover, the solvates of the oxidation couplers more particularly represent the hydrates of said couplers and/or the combination of said couplers with a linear or branched C1 to C4 alcohol such as methanol, ethanol, isopropanol or n-propanol. Preferably, the solvates are hydrates. The total amount of oxidation coupler(s), when they are present in the anhydrous solid dyeing composition according to the invention, preferably ranges from 0.1% to 45% by weight, more preferentially from 0.5% to 40% by weight, better still from 1% to 35% by weight and even better still from 5% to 35% by weight, relative to the total weight of the anhydrous solid dyeing composition. Surfactants The anhydrous solid dyeing composition according to the present invention can optionally also comprise one or more surfactants, preferably chosen from anionic surfactants, amphoteric or zwitterionic surfactants, non-ionic surfactants, cationic surfactants and mixtures thereof. For the purposes of the present invention, the term “surfactant” is intended to mean an agent comprising at least one hydrophilic group and at least one lipophilic group in its structure, and which is preferably capable of reducing the surface tension of water, and comprising in its structure, as optional repeating units, only alkylene oxide units and/or sugar units and/or siloxane units. Preferably, the lipophilic group is a fatty chain comprising from 8 to 30 carbon atoms. Preferably, the anhydrous solid dyeing composition according to the present invention comprises one or more surfactants chosen from anionic surfactants. The term “anionic surfactant” is intended to mean a surfactant comprising, as ionic or ionizable groups, only anionic groups. These anionic groups are preferably chosen from the groups C(O)OH, C(O)O—, —SO3H, —S(O)2O−, —OS(O)2OH, —OS(O)2O−, —P(O)OH2, —P(O)2O−, —P(O)O2−, —P(OH)2, ═P(O)OH, —P(OH)O−, ═P(O)O−, ═POH and ═PO−, the anionic parts comprising a cationic counterion such as those derived from an alkali metal, an alkaline-earth metal, an amine or an ammonium. As examples of anionic surfactants that may be used in the composition according to the invention, mention may be made of alkyl sulfates, alkyl ether sulfates, alkylamido ether sulfates, alkylaryl polyether sulfates, monoglyceride sulfates, alkylsulfonates, alkylamidesulfonates, alkylarylsulfonates, α-olefin sulfonates, paraffin sulfonates, alkyl sulfosuccinates, alkyl ether sulfosuccinates, alkylamide sulfosuccinates, alkyl sulfoacetates, acylsarcosinates, acylglutamates, alkyl sulfosuccinamates, acylisethionates and N-acyltaurates, polyglycoside polycarboxylic acid and alkyl monoester salts, acyl lactylates, salts of D-galactoside uronic acids, salts of alkyl ether carboxylic acids, salts of alkylaryl ether carboxylic acids, salts of alkylamido ether carboxylic acids; and the corresponding non-salified forms of all these compounds; the alkyl and acyl groups of all these compounds comprising from 6 to 24 carbon atoms and the aryl group denoting a phenyl group. These compounds may be oxyethylenated and then preferably comprise from 1 to 50 ethylene oxide units. The salts of C6 to C24 alkyl monoesters of polyglycoside-polycarboxylic acids may be chosen from C6 to C24 alkyl polyglycoside-citrates, C6 to C24 alkyl polyglycoside-tartrates and C6 to C24 alkyl polyglycoside-sulfosuccinates. When the anionic surfactant(s) are in salt form, they may be chosen from alkali metal salts such as the sodium or potassium salt and preferably the sodium salt, ammonium salts, amine salts and in particular amino alcohol salts or alkaline-earth metal salts such as the magnesium salts. Examples of amino alcohol salts that may in particular be mentioned include monoethanolamine, diethanolamine and triethanolamine salts, monoisopropanolamine, diisopropanolamine or triisopropanolamine salts, 2-amino-2-methyl-1-propanol salts, 2-amino-2-methyl-1,3-propanediol salts and tris(hydroxymethyl)aminomethane salts. Use is preferably made of alkali metal or alkaline-earth metal salts, and in particular sodium or magnesium salts. Among the anionic surfactants mentioned, use is preferably made of (C6-C24)alkyl sulfates, (C6-C24)alkyl ether sulfates comprising from 2 to 50 ethylene oxide units, in particular in the form of alkali metal, ammonium, amino alcohol and alkaline-earth metal salts, or a mixture of these compounds. It is in particular preferred to use (C12-C20)alkyl sulfates, (C12-C20)alkyl ether sulfates comprising from 2 to 20 ethylene oxide units, in particular in the form of alkali metal, ammonium, amino alcohol and alkaline-earth metal salts, or a mixture of these compounds. Even better still, it is preferred to use sodium lauryl ether sulfate, in particular those containing 2.2 mol of ethylene oxide, more preferentially (C12-C20)alkyl sulfates such as an alkali metal lauryl sulfate such as sodium lauryl sulfate. Preferably, the anhydrous solid dyeing composition according to the present invention comprises one or more surfactants chosen from amphoteric or zwitterionic surfactants. The amphoteric or zwitterionic surfactant(s) of the invention are preferably non-silicone, and are in particular derivatives of optionally quaternized aliphatic secondary or tertiary amines, in which derivatives the aliphatic group is a linear or branched chain comprising from 8 to 22 carbon atoms, said amine derivatives containing at least one anionic group, for instance a carboxylate, sulfonate, sulfate, phosphate or phosphonate group. Mention may be made in particular of (C8-C20)alkyl betaines, sulfobetaines, (C8-C20)alkylamido(C3-C8)alkyl betaines and (C8-C20)alkylamido(C6-C8)alkyl sulfobetaines. Among the amphoteric or zwitterionic surfactants mentioned above, use is preferably made of (C8-C20)alkylbetaines such as cocoylbetaine, and (C8-C20)alkylamido(C3-C8)alkylbetaines such as cocamidopropylbetaine, and mixtures thereof. More preferentially, the amphoteric or zwitterionic surfactant(s) are chosen from cocamidopropylbetaine and cocoylbetaine. Preferably, the anhydrous solid dyeing composition according to the present invention comprises one or more surfactants chosen from cationic surfactants. The cationic surfactant(s) that may be used in the composition according to the invention comprise, for example, optionally polyoxyalkylenated primary, secondary or tertiary fatty amine salts, quaternary ammonium salts, and mixtures thereof. Among the cationic surfactants that may be present in the composition according to the invention, it is more particularly preferred to choose cetyltrimethylammonium, behenyltrimethylammonium and dipalmitoylethyl-hydroxyethylmethylammonium salts, and mixtures thereof, and more particularly behenyltrimethylammonium chloride, cetyltrimethylammonium chloride, and dipalmitoylethylhydroxyethylammonium methosulfate, and mixtures thereof. Preferably, the anhydrous solid dyeing composition according to the present invention comprises one or more surfactants chosen from non-ionic surfactants. Examples of non-ionic surfactants that may be used in the composition according to the present invention are described, for example, in the “Handbook of Surfactants” by M. R. Porter, published by Blackie & Son (Glasgow and London), 1991, pp. 116-178. Examples of non-ionic surfactants that may be mentioned include: oxyalkylenated (C8-C24)alkylphenols; saturated or unsaturated, linear or branched, oxyalkylenated or glycerolated C8 to C30 alcohols; saturated or unsaturated, linear or branched, oxyalkylenated C8 to C30 amides; esters of saturated or unsaturated, linear or branched, C8 to C30 acids and of polyethylene glycols; polyoxyethylenated esters of saturated or unsaturated, linear or branched, C8 to C30 acids and of sorbitol; esters of fatty acids and of sucrose; (C8-C30)alkylpolyglycosides, (C8-C30)alkenylpolyglycosides, optionally oxyalkylenated (0 to 10 oxyalkylene units) and comprising 1 to 15 glucose units, (C8-C30)alkylglucoside esters; saturated or unsaturated oxyethylenated plant oils; condensates of ethylene oxide and/or of propylene oxide, inter alia, alone or as mixtures; N—(C8-C30)alkylglucamine and N—(C8-C30)acylmethylglucamine derivatives; aldobionamides; amine oxides; and oxyethylenated and/or oxypropylenated silicones. The surfactants containing a number of moles of ethylene oxide and/or of propylene oxide ranging advantageously from 1 to 100, more particularly from 2 to 100, preferably from 2 to 50 and more advantageously from 2 to 30. Advantageously, the non-ionic surfactants do not comprise any oxypropylene units. In accordance with a preferred embodiment of the invention, the non-ionic surfactants are chosen from oxyethylenated C8 to C30 alcohols comprising from 1 to 100 mol and more particularly from 2 to 100 mol of ethylene oxide; polyoxyethylenated esters of saturated or unsaturated, linear or branched C8 to C30 acids and of sorbitan comprising from 1 to 100 mol and better still from 2 to 100 mol of ethylene oxide. As examples of monoglycerolated or polyglycerolated non-ionic surfactants, monoglycerolated or polyglycerolated C8 to C40 alcohols are preferably used. In particular, the monoglycerolated or polyglycerolated C8 to C40 alcohols preferably correspond to formula (A8) below: R29O—[CH2—CH(CH2OH)—O]m—H (A8) in which: R29 represents a linear or branched C8 to C40 and preferably C8 to C30 alkyl or alkenyl radical; and m represents a number ranging from 1 to 30 and preferably from 1 to 10. As examples of compounds of formula (A8) that are suitable for use in the context of the invention, mention may be made of lauryl alcohol containing 4 mol of glycerol (INCI name: Polyglyceryl-4 Lauryl Ether), lauryl alcohol containing 1.5 mol of glycerol, oleyl alcohol containing 4 mol of glycerol (INCI name: Polyglyceryl-4 Oleyl Ether), oleyl alcohol containing 2 mol of glycerol (INCI name: Polyglyceryl-2 Oleyl Ether), cetearyl alcohol comprising 2 mol of glycerol, cetearyl alcohol comprising 6 mol of glycerol, oleocetyl alcohol comprising 6 mol of glycerol and octadecanol comprising 6 mol of glycerol. The alcohol of formula (A8) may represent a mixture of alcohols in the same way that the value of m represents a statistical value, which means that, in a commercial product, several species of polyglycerolated fatty alcohols may coexist in the form of a mixture. Among the monoglycerolated or polyglycerolated alcohols, it is more particularly preferred to use the C8 to C10 alcohol containing 1 mol of glycerol, the C10 to C12 alcohol containing 1 mol of glycerol and the C12 alcohol containing 1.5 mol of glycerol. Preferentially, the non-ionic surfactant used in the process of the invention in the composition is a monooxyalkylenated or polyoxyalkylenated, particularly monooxyethylenated or polyoxyethylenated, or monooxypropylenated or polyoxypropylenated, non-ionic surfactant, or a combination thereof, more particularly monooxyethylenated or polyoxyethylenated, monoglycerolated or polyglycerolated surfactants and alkylpolyglucosides. More preferably still, the non-ionic surfactants are chosen from polyoxyethylenated sorbitol esters, polyoxyethylenated fatty alcohols and alkylpolyglucosides, and mixtures thereof. More preferentially, the anhydrous solid dyeing composition according to the present invention comprises one or more surfactants chosen from anionic surfactants, non-ionic surfactants and mixtures thereof, and more preferentially from anionic surfactants and mixtures thereof. Alkaline Agents The anhydrous solid dyeing composition according to the present invention may optionally also comprise one or more alkaline agents. The alkaline agent(s) can be chosen from silicates and metasilicates such as alkali metal metasilicates, carbonates or hydrogen carbonates of alkali metals or alkaline-earth metals, such as lithium, sodium, potassium, magnesium, calcium or barium, and mixtures thereof. The alkaline agent(s) can also be chosen from ammonium salts, and in particular inorganic ammonium salts. Preferably, the ammonium salt(s) are chosen from ammonium halides, such as ammonium chloride, ammonium sulfate, ammonium phosphate, ammonium nitrate and mixtures thereof. More preferentially, the ammonium salt is ammonium chloride or ammonium sulfate. In one preferred embodiment, the anhydrous solid dyeing composition according to the present invention comprises one or more alkaline agents. Even more preferentially, the anhydrous solid dyeing composition according to the present invention comprises one or more ammonium salts, preferably chosen from ammonium chloride or ammonium sulfate, better still ammonium sulfate. The total amount of alkaline agent(s), when they are present in the anhydrous solid dyeing composition according to the present invention, preferably ranges from 0.1% to 50% by weight, more preferentially from 5% to 45% by weight and better still from 10% to 40% by weight, relative to the total weight of the anhydrous solid dyeing composition. More particularly, the total amount of ammonium(s), in particular of inorganic ammonium salts, and especially ammonium sulfate, when it (they) is (are) present in the anhydrous solid dyeing composition according to the present invention, preferably ranges from 0.1% to 50% by weight, more preferentially from 5% to 45% by weight and better still from 10% to 40% by weight, relative to the total weight of the anhydrous solid dyeing composition. Chemical Oxidizing Agents The anhydrous solid dyeing composition according to the invention may optionally also comprise one or more chemical oxidizing agents. According to the invention, the term “chemical oxidizing agent” is intended to mean an oxidizing agent other than atmospheric oxygen. The anhydrous solid dyeing composition according to the present invention may comprise one or more chemical oxidizing agents, preferably one or more anhydrous chemical oxidizing agents, and more preferentially one or more anhydrous solid chemical oxidizing agents, that is to say in the form of a powder, a paste or particles (such as balls). More particularly, the anhydrous chemical oxidizing agent(s) are chosen from (i) peroxygenated salts, for instance persulfates, perborates, peracids and precursors thereof; (ii) percarbonates of alkali metals or alkaline-earth metals, such as sodium carbonate peroxide, also known as sodium percarbonate; (iii) alkali metal bromates or ferricyanides; (iv) solid hydrogen peroxide-generating chemical oxidizing agents such as urea peroxide and polymer complexes that can release hydrogen peroxide, especially those comprising a heterocyclic vinyl monomer such as polyvinylpyrrolidone/H2O2 complexes, in particular in powder form, which are different from polymers comprising at least one heterocyclic vinyl monomer has defined below; (v) oxidases that produce hydrogen peroxide in the presence of a suitable substrate (for example glucose in the case of glucose oxidase or uric acid with uricase); (vi) mixtures thereof. According to one particular embodiment, the chemical oxidizing agent(s) are chosen from complexes of hydrogen peroxide and of polymer containing as monomer at least one heterocyclic vinyl monomer. More particularly, the heterocyclic vinyl monomer is chosen from monomers comprising a 4- to 6-membered heterocycle, optionally fused to a benzene ring and comprising from 1 to 4 identical or different intracyclic heteroatoms; the number of intracyclic heteroatoms being less than the number of ring members of the heterocycle. Preferably, the number of intracyclic heteroatoms is 1 or 2. More particularly, the heteroatom(s) are chosen from sulfur, oxygen and nitrogen, preferably from nitrogen and oxygen. In accordance with an even more advantageous embodiment of the invention, the monomer comprises at least one intracyclic nitrogen atom. The vinyl heterocycle may optionally be substituted with one or more C1 to C4 and preferably C1 to C2 alkyl groups. Preferably, the heterocyclic monomer is chosen from N-vinyl monomers. Among the monomers that may be envisaged, mention may be made of the following optionally substituted monomers: N-vinylpyrrolidone, vinylcaprolactam, N-vinylpiperidone, N-vinyl-3-morpholine, N-vinyl-4-oxazolinone, 2-vinylpyridine, 4-vinylpyridine, 2-vinylquinoline, 1-vinylimidazole and 1-vinylcarbazole. Preferably, the monomer is optionally substituted N-vinylpyrrolidone. In accordance with one particularly advantageous embodiment of the invention, the polymer is a homopolymer. However, it is not excluded to use a copolymer. In such a case, the comonomer(s) are chosen from vinyl acetate, (meth)acrylic acids, (meth)acrylamides and C1 to C4 alkyl esters of (meth)acrylic acid, which may be substituted or unsubstituted. The polymer participating in this complex is preferably water-soluble. It may have variable average molecular weights, preferably between 103 and 3×106 g/mol and more preferentially between 103 and 2×106 g/mol. It is also possible to use mixtures of such polymers. Advantageously, said complex comprises from 10% to 30% by weight, preferably from 13% to 25% by weight and more preferentially from 18% to 22% by weight of hydrogen peroxide relative to the total weight of the complex. According to an even more advantageous variant of the invention, in this complex, the mole ratio between the heterocyclic vinyl monomer(s) and the hydrogen peroxide ranges from 0.5 to 2 and preferably from 0.5 to 1. This complex is advantageously in the form of a substantially anhydrous powder. Complexes of this type are especially described in U.S. Pat. Nos. 5,008,106, 5,077,047, EP 832 846, EP 714 919, DE 4344131 and DE 195 45 380 and the other polymer complexes described in U.S. Pat. Nos. 5,008,093, 3,376,110 and 5,183,901. Examples of complexes that may be mentioned include products such as Peroxydone K-30, Peroxydone K-90 and Peroxydone XL-10 and also complexes formed with hydrogen peroxide and one of the following polymers such as Plasdone K-17, Plasdone K-25, Plasdone K-29/32, Plasdone K-90, Polyplasdone INF-10, Polyplasdone XL-10, Polyplasdone XL, Plasdone 5-630, Styleze 2000 Terpolymer and the series of Ganex copolymers, sold by the company ISP. Preferably, the composition according to the present invention can comprise one or more anhydrous solid chemical oxidizing agents chosen from urea peroxide; polymer complexes that can release hydrogen peroxide, chosen from polyvinylpyrrolidone/H2O2 complexes, which are different from polymers comprising at least one heterocyclic vinyl monomer has defined below; perborates; percarbonates; and mixtures thereof. More preferentially, the composition according to the present invention can comprise one or more anhydrous solid chemical oxidizing agents chosen from alkali metal or alkaline-earth metal percarbonates and mixtures thereof. Even better still, the chemical oxidizing agent is sodium percarbonate. The total amount of chemical oxidizing agent(s), when they are present in the anhydrous solid dyeing composition according to the present invention, preferably ranges from 1% to 20% by weight and more preferably from 5% to 15% by weight, relative to the total weight of the composition. According to one particularly preferred embodiment, the anhydrous solid dyeing composition according to the present invention does not comprise a chemical oxidizing agent. Polymers Comprising at Least One Heterocyclic Vinyl Monomer The anhydrous solid dyeing composition according to the present invention may optionally also comprise one or more polymers comprising at least one heterocyclic vinyl monomer. More particularly, the heterocyclic vinyl monomer is chosen from monomers comprising a 4- to 7-membered heterocycle, and comprising from 1 to 4 identical or different intracyclic heteroatoms, which is optionally fused to a benzene ring and/or optionally substituted; the number of intracyclic heteroatoms being less than the number of ring members of the heterocycle. Preferably, the number of intracyclic heteroatoms is 1 or 2. More particularly, the heteroatom(s) are chosen from sulfur, oxygen and nitrogen, and preferably from nitrogen and oxygen. In accordance with an even more advantageous embodiment of the invention, the monomer comprises at least one intracyclic nitrogen atom. The vinyl heterocycle may optionally be substituted with one or more C1 to C4 and preferably C1 to C2 alkyl groups. Preferably, the heterocyclic monomer is chosen from N-vinyl monomers. Among the heterocyclic vinyl monomers that may be envisaged, mention may advantageously be made of the following optionally substituted monomers: N-vinylpyrrolidone, vinylcaprolactam, N-vinylpiperidone, N-vinyl-3-morpholine, N-vinyl-4-oxazolinone, 2-vinylpyridine, 4-vinylpyridine, 2-vinylquinoline, 1-vinylimidazole and 1-vinylcarbazole. Preferably, the monomer is optionally substituted N-vinylpyrrolidone. In accordance with one particularly advantageous embodiment of the invention, the polymer is a homopolymer. However, it is not excluded to use a copolymer. The copolymer can comprise at least two distinct heterocyclic vinyl monomers as described previously, or else at least one heterocyclic vinyl monomer, as described previously, and at least one monomer that is different from the heterocyclic vinyl monomers, as described previously. In the latter case, the comonomer(s) are preferably chosen from vinyl acetate, (meth)acrylic acids, (meth)acrylamides and C1 to C4 alkyl esters of (meth)acrylic acid, which may be substituted or unsubstituted. The polymer comprising at least one heterocyclic vinyl monomer may be crosslinked or non-crosslinked. The polymer comprising at least one heterocyclic vinyl monomer according to the invention is preferably water-soluble. It may have variable average molecular weights, preferably between 103 and 3×106 g/mol, and more preferentially between 103 and 2×106 g/mol. It is also possible to use mixtures of such polymers. Preferably, the polymer comprising at least one heterocyclic vinyl monomer is the crosslinked or non-crosslinked vinylpyrrolidone homopolymer. The total amount of polymer(s) comprising at least one heterocyclic vinyl monomer, when they present in the anhydrous solid dyeing composition according to the present invention, preferably ranges from 5% to 70% by weight, more preferentially from 10% to 60% by weight and better still from 10% to 35% by weight, relative to the total weight of the anhydrous solid dyeing composition. Additives The anhydrous solid dyeing composition according to the present invention may also optionally comprise one or more additives, different from the compounds of the invention and among which mention may be made of cationic, anionic, non-ionic or amphoteric polymers or mixtures thereof, antidandruff agents, anti-seborrhoea agents, agents for preventing hair loss and/or for promoting hair regrowth, vitamins and provitamins including panthenol, sunscreens, mineral or organic pigments, sequestrants, plasticizers, solubilizers, acidifying agents, mineral or organic thickeners, in particular polymeric thickeners, opacifiers or nacreous agents, antioxidants, hydroxy acids, fragrances, preservatives, pigments and ceramides. Needless to say, those skilled in the art will take care to select this or these optional additional compound(s) such that the advantageous properties intrinsically associated with the anhydrous solid dyeing composition in accordance with the invention are not, or are not substantially, adversely affected by the envisaged addition(s). The above additives may generally be present in an amount, for each of them, of between 0 and 20% by weight relative to the total weight of the anhydrous solid dyeing composition. Ready-to-Use Composition The present invention also relates to a ready-to-use composition comprising an anhydrous solid dyeing composition, as defined above, one or more chemical oxidizing agents as defined previously, and optionally a cosmetically acceptable medium. The term “ready-to-use composition” is intended to mean a composition intended to be applied directly to the keratin fibres without being premixed with another composition. The keratin fibres on which said ready-to-use composition is applied may be dry or wet. The total amount of oxidation base(s) present in the ready-to-use composition according to the present invention preferably ranges from 0.05% to 15% by weight, more preferentially from 0.1% to 10% by weight, and better still from 0.5% to 5% by weight, relative to the total weight of the ready-to-use composition. The total amount of metabisulfite(s), preferably of sodium metabisulfite, present in the ready-to-use composition according to the invention preferably ranges from 0.01% to 15% by weight, more preferentially from 0.05% to 10% by weight, and better still from 0.1% to 5% by weight, relative to the total weight of the ready-to-use composition. The total amount of coupler(s), when they are present in the ready-to-use composition according to the invention, preferably ranges from 0.05% to 15% by weight, more preferentially from 0.1% to 10% by weight, and better still from 0.5% to 5% by weight, relative to the total weight of the ready-to-use composition. Preferably, the ready-to-use composition according to the present invention comprises one or more anhydrous solid chemical oxidizing agents chosen from urea peroxide; polymer complexes that can release hydrogen peroxide, chosen from polyvinylpyrrolidone/H2O2 complexes; perborates; percarbonates; and mixtures thereof. More preferentially, the ready-to-use composition according to the present invention comprises one or more anhydrous solid chemical oxidizing agents chosen from percarbonates and mixtures thereof. Even better still, the chemical oxidizing agent is sodium percarbonate. The total amount of chemical oxidizing agent(s) present in the ready-to-use composition according to the present invention preferably ranges from 0.1% to 30% by weight, more preferentially from 1% to 20% by weight, and better still from 5% to 15% by weight, relative to the total weight of the ready-to-use composition. Process for preparing the ready-to-use composition According to a first embodiment, the ready-to-use composition results from the mixing of an anhydrous solid dyeing composition as defined previously comprising one or more chemical oxidizing agents, preferably one or more anhydrous solid chemical oxidizing agents as defined previously, with a cosmetically acceptable medium. According to a second embodiment, the ready-to-use composition results from the mixing of an anhydrous solid dyeing composition as defined previously, with one or more anhydrous solid chemical oxidizing agents as defined previously and a cosmetically acceptable medium. According to this second embodiment, the ready-to-use composition is prepared by mixing the anhydrous solid dyeing composition and the anhydrous solid chemical oxidizing agents. Then, in a second step, a cosmetically acceptable mixture is added, in a sufficient amount, to the mixture of solid compositions previously obtained, in order to obtain the ready-to-use composition. According to a third embodiment, the ready-to-use composition results from the mixing of an anhydrous solid dyeing composition as defined previously, with a liquid oxidizing agent comprising one or more chemical oxidizing agents. According to this third embodiment, the ready-to-use composition is prepared by mixing the anhydrous solid dyeing composition with a sufficient amount of liquid oxidizing composition. The term “liquid” is intended to mean a composition having a viscosity of less than or equal to 2 Pa·s, preferably less than or equal to 1 Pa·s and even better still less than or equal to 0.1 Pa·s at a temperature of 25° C., at atmospheric pressure (1.013×105 Pa), and at a share rate of 1 s−1. In other words, when the chemical oxidizing agents are solid and anhydrous, a cosmetically acceptable medium is necessary in order to obtain the ready-to-use composition. Furthermore, when the chemical oxidizing agents are present in a liquid oxidizing composition, the cosmetically acceptable medium is not necessary in order to obtain the ready-to-use composition. Cosmetically Acceptable Medium The term “cosmetically acceptable medium” is intended to mean, according to the present application, a medium that is compatible with keratin fibres, in particular human keratin fibres such as the hair. The cosmetically acceptable medium is constituted of water or of a mixture of water and of one or more organic solvents. Examples of organic solvents that may be mentioned include linear or branched C2 to C4 alkanols, such as ethanol, isopropanol, tert-butanol or n-butanol; glycerol; polyols and polyol ethers, for instance 2-butoxyethanol, propylene glycol, hexylene glycol, dipropylene glycol, propylene glycol monomethyl ether, diethylene glycol monomethyl ether and monoethyl ether, and also aromatic alcohols or ethers, for instance benzyl alcohol or phenoxyethanol, and mixtures thereof. Thickening Polymers The ready-to-use composition according to the present invention may optionally also comprise one or more thickening polymers. Advantageously, the thickening polymer(s) are chosen from the following polymers: (a) non-ionic amphiphilic polymers comprising at least one fatty chain and at least one hydrophilic unit; (b) anionic amphiphilic polymers comprising at least one hydrophilic unit and at least one fatty-chain unit; (c) crosslinked acrylic acid homopolymers; (d) crosslinked homopolymers of 2-acrylamido-2-methylpropanesulfonic acid, and crosslinked acrylamide copolymers thereof which are partially or totally neutralized; (e) ammonium acrylate homopolymers or copolymers of ammonium acrylate and of acrylamide; (f) dimethylaminoethyl methacrylate homopolymers quaternized with methyl chloride or dimethylaminoethyl methacrylate copolymers quaternized with methyl chloride and acrylamide; and (g) polysaccharides such as: (g1) scleroglucan gums (biopolysaccharide of microbial origin); (g2) gums derived from plant exudates, such as gum arabic, ghatti gum, karaya gum or gum tragacanth; (g3) celluloses and derivatives; (g4) guar gums and derivatives; or (g5) starches or derivatives. It should be noted that, in the case of the present invention, the thickening polymers have a role regarding the viscosity of the ready-to-use composition. According to the invention, amphiphilic polymers are more particularly hydrophilic polymers that are capable, in the medium of the composition, and more particularly in an aqueous medium, of reversibly combining with each other or with other molecules. Their chemical structure more particularly comprises at least one hydrophilic group and at least one hydrophobic group. The term “hydrophobic group” is intended to mean a radical or polymer bearing a saturated or unsaturated, linear or branched hydrocarbon-based chain, comprising at least 8 carbon atoms, preferably at least 10 carbon atoms, more preferentially from 10 to 30 carbon atoms, in particular from 12 to 30 carbon atoms and even better still from 18 to 30 carbon atoms. Preferably, the hydrocarbon-based group is derived from a mono functional compound. By way of example, the hydrophobic group may be derived from a fatty alcohol such as stearyl alcohol, dodecyl alcohol or decyl alcohol. It may also denote a hydrocarbon-based polymer, such as, for example, polybutadiene. According to a first embodiment, the thickening polymer(s) are in anhydrous solid form and are present in the anhydrous solid dyeing composition. According to another embodiment, the thickening polymer(s) are present in the liquid oxidizing composition, as defined previously. According to another embodiment, the thickening polymer(s) are present in the cosmetically acceptable medium. Packaging article A subject of the present invention is also a packaging article comprising: i) an envelope defining at least one cavity, the envelope comprising water-soluble and/or liposoluble fibres, preferably water-soluble fibres; and ii) an anhydrous solid dyeing composition as defined previously; it being understood that the anhydrous solid dyeing composition is in one of the cavities defined by the envelope i). The term “water-soluble” is intended to mean soluble in water, in particular in a proportion of at least 10 grams per litre of water, preferably at least 20 g/l and better still at least 50 g/l, at a temperature of less than or equal to 35° C. The term “liposoluble” is intended to mean soluble in a liquid fatty substance as defined below, in particular in a proportion of at least 10 grams per litre of liquid fatty substance, in particular in a plant oil or mineral oil such as liquid petroleum jelly, preferably at least 20 g/l in a liquid fatty substance, better still at least 50 g/l in a fatty substance, at a temperature of less than or equal to 35° C. The term “temperature of less than or equal to 35° C.” is intended to mean a temperature not exceeding 35° C. but greater than or equal to 0° C., for example ranging from more than 1 to 35° C., better still from 5 to 30° C. and even better still from 10 to 30° C. or 10 to 20° C. It is understood that all the temperatures are given at atmospheric pressure. The packaging article according to the invention is preferably water-soluble or liposoluble at a temperature of less than or equal to 35° C. The packaging article can comprise one or more cavities, at least one of which contains the anhydrous solid dyeing composition as defined previously. According to one particular embodiment, the packaging article comprises a first cavity containing an anhydrous solid dyeing composition, as defined previously, and not comprising chemical oxidizing agent, and a second cavity containing one or more anhydrous solid chemical oxidizing agents, as defined previously. The packaging article preferably comprises only one cavity. According to another particular embodiment, the packaging article comprises a cavity containing an anhydrous solid dyeing composition as defined previously, said composition also comprising one or more chemical oxidizing agents, as defined previously, preferably one or more anhydrous solid chemical oxidizing agents. The envelope may be constituted of a sheet constituted of water-soluble and/or liposoluble fibres, which is folded on itself, or of a first sheet constituted of water-soluble and/or liposoluble fibres and covered with a second sheet also constituted of water-soluble and/or liposoluble fibres. The sheet folded on itself or the two sheets are then assembled hermetically in such a way that the anhydrous solid dyeing composition according to the present invention, and optionally the anhydrous solid oxidizing agent(s), cannot diffuse to the exterior. The anhydrous solid dyeing composition, and also optionally the anhydrous solid chemical oxidizing agent(s), are thus hermetically enveloped by the envelope i). The anhydrous solid dyeing composition, and also the anhydrous solid chemical oxidizing agent(s), when they are present, are distinct from the sheet or from the envelope. Such an envelope is different from the thin water-soluble or liposoluble films into which the anhydrous solid dyeing composition, and also optionally the anhydrous solid chemical oxidizing agent(s), would be incorporated in the sheet(s) forming the envelope. Relative to these water-soluble or liposoluble thin films, the envelope according to the invention has the advantage of allowing the
{'bibliographic_information': "{'country': 'US', 'doc-number': '16471636', 'kind': 'B2', 'date': '20171219', 'invention_title': 'Anhydrous solid composition for dyeing keratin fibres comprising a metabisulfite'}", 'source_file': './grant/2021/ipg210105.xml', 'source_data': 'USPTO-Grant'}