The present invention relates to a solid solution consisting of 60-90 mol-% of an asymmetric 2,5-dihydro-1,4-diketopyrrolo�3,4-c!pyrrole of the formula (I) ##STR1## in which G.sub.1 and G.sub.2 independently of one another are different substituted or unsubstituted aromatic radicals or pyridyl, but the volume of all substituents on G.sub.1 and G.sub.2 is greater than that of two methyl groups; and 40-10 mol-% of a 2,5-dihydro-1,4-diketopyrrolo�3,4-c!pyrrole of the formula (IIa) ##STR2## in which G.sub.3 and G.sub.4 independently of one another are different substituted or unsubstituted aromatic radicals or pyridyl, but the volume of all substituents on G.sub.3 and G.sub.4 is less than that of two butoxy groups; or 40-10 mol-% of a quinacridone of the formula (IIb) ##STR3## in which R.sub.9 is hydrogen, halogen, C.sub.1 -C.sub.4 alkyl or C.sub.1 -C.sub.4 alkoxy; which is a single-phase solid solution having the crystal structure of the asymmetric 2,5-dihydro-1,4-diketopyrrolo�3,4-c!pyrrole of the formula (I).