The present invention relates to a cosmetic process for treating keratin materials, in particular keratin fibers, preferably human keratin fibers such as the hair, using a) one or more direct dyes which are preferably neutral and b) one or more particular aliphatic ammonium salts, and to a composition comprising ingredients a) and b). The cosmetic process for treating keratin materials of the invention makes it possible to obtain a good color build-up up and, furthermore, the colors are vivid and chromatic.