The present application relates to a process for preparing dyed plastics or polymeric dye particles characterized in that it comprises mixing a high molecular weight organic material with a coloristically effective amount of at least one dye of the formula where R1 is hydrogen, hydroxyl or a radical -NHCO-R6, where R6 is C1-C6alkyl or unsubstituted or C1-C4alkyl- or halogen- substituted phenyl, R2 is hydrogen, hydroxyl or a radical W-R7, where W is -NHCO- or -S- and R7 is C1-C6alkyl or unsubstituted or C1-C4alkyl- or halogen-substituted phenyl, R3 is hydrogen, R4 is hydrogen or hydroxyl, R5 is hydrogen, or R3 and R4 combine to form the radical of the formula, where the rings A and B may independently bear further substituents.