Particularly preferred as color developers are N,N-dialkyl-p-phenylenediamines, of the general formula wherein each of R1 and R2, independently of the other, is an alkyl group, preferably with from 1 to about 4 carbon atoms, or a substituted alkyl group, wherein the benzene ring can be substituted, and wherein 2 or more substituents can be joined together to form additional rings.