Specific examples of preferred dyes conform to the formulae (Ia), (Ib) and (Ic) ##STR3## where, in these formulae, in each case independently of the other substituents, the two R3 are hydrogen, C1 -C3 -alkyl, C1 -C2 -alkoxy-C2 -C3 -alkyl or phenyl, in particular methyl, ethyl, C1 -C2 -alkoxy-C2 -C3 -alkyl or phenyl;