Abstract: A process and composition for coloring substances and tissues containing keratin, by means of an agent or co-reactant having a ketone or aldehyde function. An amino-acid, in particular a thio-amino-acid, is contacted with the substance to be colored. The composition can be in the form of a solution or a suspension containing the amino-acid, applicable separately from or together with the co-reactant of known type based on a ketone or aldehyde. The invention is useful in cosmetics, for tanning the skin or coloring the hair or nails. It is also applicable to the coloration of various articles, in particular hair in general, feathers, horn, shell etc.