The present invention relates to an azo dye of formula I, II or III: (formula I, II or III); wherein R1 and R2 are each independently of the other one or more divalent groups selected from alkylene, arylene, aralkylene and cycloalkylene, which may be interrupted by —O—, —S—, —NH—, —NR8—, —COO—, —CONH— or —CONR9—, wherein R8 and R9 are alkyl or aryl, X is —O— or —NH—, R3 is —CN or —CONH2, R4 is methyl or trifluoromethyl and R5, R6, R7 and R8 are each independently of the others hydrogen, halogen or —CN, to a process for the preparation thereof and to the use thereof in a method of producing mass-coloured plastics or polymeric colour particles.