Cosmetic hair post-treatment agent for improving washing fastness of coloring processes produced on hair and for color fixing, the post-treatment agent containing, in a cosmetic carrier, at least one acetylpyridinium derivative of formula (I) wherein R1 is a C1 to C6 alkyl group, a C2 to C6 alkenyl group, a C2 to C6 hydroxyalkyl group, a C1 to C6 alkoxy-C2 to C6 alkyl group, a carboxy-C2 to C6 alkyl group, an aryl-C1 to C6 alkyl group, a heteroaryl-C1 to C6 alkyl group, an aryl group, or a heteroaryl group, and X− is a physiologically acceptable anion.