Abstract: Hydraulic coupler compounds corresponding to the formula: ##STR1## in which R.sub.1 denotes a hydrogen atom or a C.sub.1 -C.sub.4 alkyl radical; R.sub.2 and R.sub.3, which may be identical or different, denote a hydrogen atom, a C.sub.1 -C.sub.4 lower alkyl radical, a carboxyl radical or an alkoxycarbonyl radical; X denotes a C.sub.1 -C.sub.4 lower alkyl radical, a C.sub.1 -C.sub.18 alkoxy radical, a halogen atom, a C.sub.1 -C.sub.20 acyloxy radical, an acetylamino group, a trimethylsilyloxy group or a dialkyl(C.sub.1 -C.sub.4)aminomethyl group, the OH group occupying the 6- or 7-positions, as well as their salts, are combined with oxidation dye precursors for dyeing keratinous fibers.