A colouring agent for keratin fibres, in particular human hair, comprising at least one compound selected from oxidation dye precursors and direct dyes, 0.1-5% of at least one dicarboxylic acid having 2 to 10 carbon atoms and/or a salt thereof, at least one keratin hydrolysate having an average molecular weight in the range 2000 to 7000 Daltons, 20-95% water and 0-0.1% of peroxide compounds. The dicarboxylic acid is preferably selected from succinic acid, malic acid, and mixtures thereof. The colouring agent may additional comprise at least one amino acid selected from arginine, lysine and histidine. A method for oxidative dyeing of keratin fibres comprising the steps of providing a composition (A) comprising the composition defined for the colouring agent and providing a composition (B) comprising at least one peroxide compound, mixing (A) and (B), applying the mixture to the keratin fibres and rinsing after an exposure time of 0.1 to 60 minutes. Composition (B) preferably contains water and has a pH value in the range of 2.5 to 6.5.
