Why does phosphoric acid mask the colour of iron(III) complex in water?
When phosphoric acid is added to an aqueous solution of iron (III), the ligands in the yellow complexes $\ce{[Fe(OH)(H2O)5]^2+}$ and $\ce{[Fe(OH)2(H2O)4]+}$ ( $\ce{[Fe(H2O)6]^3+}$ are weakly colored but fairly acidic), are successively substituted by $\ce{H2PO4^-}$ and $\ce{HPO4^2-}$. This yields colorless hydrogenphosphatoiron(III) complexes, like $\ce{[Fe(H2PO4)(H2O)5]^2+}$ and the water-free $\ce{Fe(H2PO4)3}$ complex, depending on the phosphoric acid concentration (source). In less acidic solutions, where $\ce{PO4^3-}$ is present, the colorless bis(phosphato)ferrate(III) $\ce{[Fe(PO4)2]^3-}$ is formed (source).