The present invention relates to a composition for dyeing a keratinous fiber, comprising, a) at least one oxidation base or direct dye or a mixture thereof, b) at least one particular polycarboxylic acid compound or a salt thereof, and c) at least one suitable medium. The invention also relates to a ready-to-use composition comprising the above-mentioned composition and an oxidizing composition including at least one oxidizing agent in a suitable medium for dyeing. The invention also relates to a process for dyeing a keratinous fiber, comprising: mixing the composition according to the invention and optionally an oxidizing composition including at least one oxidizing agent in a suitable medium for dyeing; applying mixed composition to said keratinous fiber shortly after mixing; leaving the mixed composition on the keratinous fiber for sufficient time to obtain a desired coloration; rinsing the keratinous fiber to remove the mixed composition from the keratinous fiber; optionally washing and rinsing the keratinous fiber; and optionally drying the keratinous fiber. Finally, the invention relates to a device for implementing the process.