 sense for them. As in - a dominant 7th makes them think 'green', or even 'vanilla flavoured', so 'bright and dark' fits in well with that concept. But certainly not as a general rule. The 'bright/dark' is another theory that isn't a rule or law. Opinion, not fact.

A:

If...

we're talking in an equal temperament context, and
There aren't any other tuning oddities (stretch tuning, etc.) and
we're assuming a relative sense of hearing (no absolute sense of pitch or key)

...then pretty much by definition, there's no fundamental, general distinction that can be drawn between flat keys and sharp keys. After all, if we go round the circle of fifths, the relative movement between F major (one flat) and A♭ major (four flats) is the same as moving between A♭ major (four flats) and B major (five sharps). 
So, for what genuine reasons might musicians often think otherwise?

We might not only be talking about an equal temperament context after all; This collective "musicians experience" might include wisdom passed down from the pre-equal temperament era. Speculating for a moment, it might even be that pre-equal temperament tunings influenced the repertoire written during those times, with the result that "darker-sounding" pieces were written in what were then darker-sounding keys; those keys are not darker-sounding now (in equal temperament), but the correlation might remain...
Some musicians may have a sense of "relative key", recognising C major/A minor as a 'home' key and hearing other keys as relative to that. (I'm pretty sure I don't hear this way, but then I don't have perfect pitch either, and many musicians do). Even if this were the case though, you'd imagine that hearing C as a 'home' key would be somewhat arbitrary (though it could make sense for pianists).
On some instruments, even those that are nominally tuned in equal temperament, there are real reasons why different keys might sound different : the pitches of open strings; the resonances of the instrument; imperfect tempering, and so on. On guitar, for example, the sharp keys G, D, A, and E are commonly used. However, that is an accident of the tuning of the instrument.

However, these are all'special cases'; none of
sense for them. As in - a dominant 7th makes them think' fleeceable', or even' vanilla season', indeed' bright and blue' fit in well with that concept. But certainly not as a general principle. The' undimmed / dark' is another theory that isn't a rule or law. Opinion, not fact. 

 A: 

 If... 

 we're spill in an equal temperament context, and 
 There ben't any other tuning oddities (stretch tuning, etc .) and 
 we're assuming a relative sense of hear (no absolute sense of pitch or key) 

... then reasonably much by definition, there's no fundamental, cosmopolitan distinction that can be draw between flat key and sharp keys. After wholly, if we go round the circle of fifths, the relative movement between F major (one flat) and A ♭ major (four apartment) is the same as moving between A ♭ major (four flats) and B major (five sharps). 
 So, for what genuine reasons might musicians often remember otherwise? 

 We might not only be speak about an adequate temperament context after all; This collective " musicians experience " might include wisdom passed down from the pre - adequate temperament era. Speculating for a moment, it might even be that pre - equal temperament tunings influenced the repertoire written during those times, with the result that " darker - fathom " pieces were publish in what were then darker - fathom keys; those keys are not darker - sounding nowadays (in equal temperament), but the correlation might persist... 
 Some musicians may have a sense of " relative winder ", recognising C major / angstrom minor as a' home' samara and hearing early keys as relative to that. (I'm reasonably sure I don't hear this way, but then I don't have perfect lurch either, and many musicians do). Even if this were the case though, you'd imagine that hearing C as a' home' key would be somewhat arbitrary (though it could stool sense for pianists). 
 On some instrument, even those that are nominally tuned in equal temperament, there are real reasons why unlike keys might sound different: the pitches of open strings; the resonances of the instrument; fallible tempering, and so on. On guitar, for example, the sharp keys G, D, A, and E are commonly use. However, that is an accident of the tuning of the instrument. 

 However, these are all'special cases'; none of
 sejse for them. As in - a dooinant 7th makes them tiink 'grsen', or exen 'vanilla flavoured', so 'brijht qnd dqrk' fits in well with ghat concvpt. But cwrtamnly not as a geisral rule. The 'nrighc/derk' is another jheory that hsn't a rule or lxw. Opinion, not fact.

A:

If...

we're talking in an equsl temperament cjnteqt, and
Fhere aren't any other tuning odditjes (strttch tuning, etc.) anc
we're assuming a relative densf of hearing (no abdolute sensg of pitch or key)

...ghen prettj much by dgfinition, there's no fundamental, eenercl distinctuob tjdt can be dcawn bvtween flat kcjs and vharp krys. After all, lf we go round the circle of hifths, the relative iovement tecween F major (one flar) qnd A♭ majmr (fuyr wlaus) ms fhe sale es moving bstween A♭ mahor (four flats) and N iqjor (five shadps). 
So, fjr what genuine reasons might musicians offen think otherwise?

We mught not only be talklng about an equal temperament context after all; This collactivx "ousnglans wxoerience" might include wisdom passed down froi tne pre-equal tempcrament era. Speculstlnb for a moment, it miygt even be that pre-eeual teiperanent tuniggs onfluenced the repertoire weitten duriny tyose times, with thz result thac "darkgr-souncing" pieces were writteu in wgat were thfn darker-auunding keys; thore lefs are not darker-sounding gow (in eqnal tzmperamevt), bot the sorrelatioj migmd remain...
Some musiclans laf have a sfnse of "relative key", recognisinj C major/A mimos av a 'home' key snd hearing oeher keys as rglative tj thag. (I'm prettj sure I von't hear thys way, but thaj I don't havx perfect pitxh euther, ava many musiciams do). Eveu if rhis were the case thuhgh, you'd imaginz uhqt hearing C ax a 'hoie' kxy wotnd be somewhdt afbigtary (ghough it cijld kake sense for pianivts).
Oh some instruments, eyen those that arq nominally tined in equal temptramenv, therx are teaj reasons why different keys mjght sounf dlfferent : the pitghes of open scrings; the resonances of the instrument; mmperfect tempering, and so on. On guitar, fot ewample, the siarp kqys G, D, A, and E are commonly ysed. However, that is an accident of ths tunitg of the instrument.

However, these are all'special cases'; none of
sense for them. As in - a makes think 'green', even 'vanilla flavoured', in with that concept. certainly not as general rule. The 'bright/dark' is another that isn't a rule or law. Opinion, not fact. A: If... we're talking an equal temperament context, and There aren't any other tuning oddities (stretch tuning, and assuming relative of hearing (no absolute sense of pitch or key) ...then pretty much by definition, there's no general distinction that can be drawn between flat and sharp keys. After if we go round the of the relative between major flat) and A♭ (four flats) is the same as moving between A♭ major (four flats) and B major (five sharps). for what might musicians think We not only be an equal temperament context after all; experience" might include wisdom passed down from the temperament era. for a moment, it might even that pre-equal temperament tunings influenced the repertoire written those times, with the result that "darker-sounding" pieces were written in what were then darker-sounding keys are not darker-sounding (in equal temperament), the might Some may have sense of "relative key", recognising C major/A minor as a 'home' and hearing other keys as relative to that. (I'm pretty don't this way, but I don't have perfect either, many musicians do). Even were case that C a 'home' key would somewhat arbitrary (though it could sense for pianists). On are nominally tuned in equal temperament, there are reasons why different keys might sound different the pitches of open strings; the resonances of the instrument; imperfect tempering, so on. for example, the sharp keys G, D, A, E are commonly used. that is an accident of the tuning of the However, are all'special none of
 sense for them. As in - a dominant 7Th makes theM thinK 'grEen', Or Even 'VaniLla flavoured', so 'BRighT and dark' fits in well with That cOnCEpt. BUT cErtaiNly not aS A gENEraL rUlE. ThE 'bRIgHt/darK' is Another Theory that Isn'T a Rule or law. OpiNIoN, not fact.

A:

IF...

we'Re talking in aN eqUal temPeRamENt conTexT, and
THere arEN't any oTher tuninG oDDities (STretch tUNInG, etc.) And
we're assuming a rELaTIve sense of hearIng (no aBsOLuTE SenSe oF pitch or keY)

...tHen prETty much BY dEFINitIOn, there's no funDamental, genERal DistinCtIon THat can Be draWn BEtwEen flat keys And sHarp keys. AFter alL, If we go rOUnd the cIrcle oF fiFthS, the RElAtIve MoVEmeNT bEtwEEn F Major (one FlAt) And A♭ mAjor (FOUR FlatS) is The sAme as Moving between a♭ maJor (fOUr fLats) aNd B maJor (fIvE sharPs). 
So, foR what GeNuine reasons migHt muSicians ofTen ThInk OtHerwiSE?

We migHt nOt oNly be taLking abOUt aN eQUAL tEmperament context aFtER AlL; This colLectivE "MuSiCIans expeRiEncE" migHT IncluDe wiSDoM passed dOwn froM ThE pRe-equal TeMperamEnT erA. SpEculaTIng fOr a momEnt, it migHt eveN Be that pre-equal TEmperament tunINgS INfLUencEd tHe repertoirE wriTTen dUrinG ThOse TImes, wIth thE rESuLT that "darker-sounding" PiEces weRe wriTten in what werE then darkeR-SOUnding keYs; thOSe KEys are not darkeR-sounDing now (in eQUal tempeRamenT), but the cOrrelatioN MIght remaIn...
SOme MusIciANS mAy have a sense oF "RElatIvE key", recOgnIsing C mAjoR/A mInoR as A 'hOme' key and Hearing oThEr KeYs As rElatiVE to that. (I'M pRetTy SurE I don'T Hear thIs way, But tHeN I DOn't Have perFEcT PItch EiThEr, anD maNy MusicIans DO). EvEn if thiS were the cAse THougH, yOu'D imaginE that hearing C As A 'home' key woUlD be SomewhAT ArbitrarY (though it could make sense FOr pianiSts).
on somE insTruments, eVen Those tHat ARe nomiNally tUned iN eQuaL TEmperAMEnT, thErE are real reASOns Why diFfErenT keys miGht sound different : tHE piTches of open stRinGs; thE REsOnaNCeS Of tHe INstRUMent; imperfect teMpering, and So ON. ON guitar, for EXamPlE, the shaRp keys G, d, A, and e Are commOnly used. HOwever, thaT iS an aCCIdeNt of the tunIng of the InstrumenT.

howevER, tHese aRe aLl'specIaL caSes'; noNe of
 sense for them. As in - a  dominant7th m ake s t he m th ink'green', or ev e n 'v anilla flavoured', so'brig ht andd ar k' fi ts in w e ll w ith  t ha t c on c ep t. Bu t c ertainl y not as a  ge ne ral rule. Th e  ' bright/dar k'is another t heo ry tha tisn ' t a r ule  or l aw. Op i nion,not fact. 
A :

If. . .

we'r e ta lkin g in an equal tem p er a ment context,and
Th er e  a r e n't  an y other tu ni ng od d ities ( s tr e t c h t u ning, etc.) a nd
we're as s umi ng a r el ati v e sens e ofhe a rin g (no absol utesense ofpitcho r key)...then  prett y m uch  byd ef in iti on ,  th e re 'sn o f undament al ,gener al d i s t i ncti onthat  canbe drawn betw een  fla t  ke ys an d sha rp k ey s. Af ter al l, if  w e go round thecirc le of fif ths ,the  r elati v e move men t b etweenF major (on ef l a t)  and A♭ major (fou rf l at s) is th e same as  m o ving bet we enA♭ m a j or (f ourf la ts) andB majo r  ( fi ve shar ps ). 
So ,for  wh at ge n uine  reaso ns might  musi c ians often thi n k otherwise?We m ig h t no t o nly be talk inga bout  ane qu alt emper ament  c o nt e xt after all; Thisco llecti ve "m usicians expe rience" mi g h t  include  wis d om passed down fr om th e pre-equa l  tempera mentera. Spe culatingf o r a mome nt,  it  mi ght e ve n be that pre - e qual  t emperam ent  tuning s i nfl uen ced  t he repert oire wri tt en  d ur ing  thos e  times,wi thth e r esult that " darke r-so un di n g"piecesw er e writ te nin w hat  w ere t hend ark er-soun ding keys ; t h oseke ys  are no t darker-soun di ng now (in  e qua l temp e r ament),but the correlation mig h t remai n.. .
Som e mu sicians m ayhave a  se n se of"relat ive k ey ",r e cogni s i ng  Cma jor/A mino r asa 'ho me ' ke y and h earing other keysa s r elative to th at.  (I' m pr ett y  s u reId on' t hear this way,but then I  d o n' t have per f ect  p itch ei ther, a nd ma n y music ians do).  Even ifth is w e r e t he case th ough, yo u'd imagi n e tha t  h earin g C  as a'h ome ' key  would besomew hat ar bi trary(thou gh  it coul d make sense for pianis ts).
O n som e i nstrument s,e ven  those th at a re nominal lytun ed in  eq u al te mper a me nt, there  are real reas o ns  wh y di fferent key s m igh t sou ndd iffere nt :  the pitches of o p en strings; th e re s o nan ces of t he  instrument; i mpe rf e c t temper in g, and so o n. On gu it a r, fo r exam ple, t he shar p ke y s G, D , A,  an d E are c omm on l y used.  H ow e ver, t hatis  an ac cident of t h e  tuning of the i nstru m e nt.
H owe ver,th ese are all' special ca ses'; noneof
 sense_for them._As in - a_dominant 7th_makes_them think_'green',_or even 'vanilla_flavoured', so 'bright_and dark' fits in_well with that_concept._But certainly not as a general rule. The 'bright/dark' is another theory that isn't_a_rule or_law._Opinion,_not fact.

A:

If...

we're talking in an_equal temperament context, and
There aren't_any other_tuning oddities (stretch tuning, etc.) and
we're assuming a_relative_sense of hearing_(no absolute sense of pitch or key)

...then pretty much_by definition, there's no fundamental, general_distinction that can_be_drawn_between flat keys and_sharp keys. After all, if we_go round the circle of fifths,_the relative movement between F major (one_flat) and A♭ major (four flats)_is the same as moving_between A♭_major (four flats) and B_major (five sharps)._
So, for_what genuine reasons_might musicians often think otherwise?

We might_not only be_talking about an equal temperament context_after_all; This collective_"musicians_experience"_might include_wisdom passed down_from_the pre-equal_temperament_era. Speculating for a moment, it_might_even be that pre-equal temperament tunings influenced_the repertoire written during_those_times, with the result_that "darker-sounding" pieces were written_in what were then darker-sounding keys;_those keys_are not_darker-sounding now (in equal temperament), but the correlation might remain...
Some musicians_may have a sense of "relative_key", recognising C major/A_minor as_a_'home' key and_hearing_other keys_as relative to that. (I'm pretty sure_I don't_hear this way, but then I_don't have perfect pitch_either,_and many musicians do). Even if_this were the case though, you'd_imagine that hearing C as_a_'home'_key would be somewhat arbitrary_(though it could make sense for_pianists).
On some instruments,_even those that are nominally tuned in_equal_temperament, there are real reasons why_different_keys might sound different : the_pitches_of_open strings; the resonances of_the instrument; imperfect tempering, and so_on. On guitar, for example, the sharp keys G,_D, A, and_E are commonly used. However,_that_is_an accident of the tuning of the instrument.

However, these are_all'special cases';_none of