Diketopyrrolopyrroles of the formula (I): wherein R1, R2, R3 and R4 independently of one another are a C1–C4 alkyl radical or a substituted or unsubstituted phenyl radical, wherein the phenyl radical can be substituted by 1, 2, 3 or 4 substituents from the group C1–C4 alkyl, C1–C4 alkoxy, CN, F, Cl, Br, NO2, CF3, S—C1–C4 alkyl, phenyl or (C1–C2)alkylenephenyl, with the proviso that at least one of the radicals, R1, R2, R3, or R4, is one of the stated substituted or unsubstituted phenyl radicals.