Process for preserving the dyeing capacity of 5, 6-dihydroxyindole and of its and derivatives in an aqueous medium, composition and dyeing process

A composition for dyeing keratinous fibers comprising an aqueous medium containing a dyeing agent consisting of an indole having the formula

• wherein
• R1, R2, R3 and R4, each independently, represent hydrogen or C1-C4 lower alkyl with the proviso that R1, R2 and R3 represent hydrogen when R4 represents alkyl and
• a pH-regulating agent present in an amount sufficient to adjust the pH of said composition to a pHC value ranging from 5 to 10, said pH-regulating agent containing two components (A) and (B), said component (A) being triethanolamine and said component (B) being a pH-regulating agent having a dissociation constant, pKCB, in an aqueous solution at 25° C., of 2<pKCB≦pHC, the weight ratio of said component (A) to said component (B) being greater than 1 and the molar ratio of said component (A) to said component (B) being greater than or equal to 2 and less than 15.

The composition of claim which also includes a thickening agent present in an amount ranging from 0.5 to 3 percent by weight based on the total weight of said composition.