The present invention relates to an azo dye of formula (I) wherein R1 and R2 are each independently of the other —NH—(CH2CH2)—OH, —NH—CH2—CHOH—CH2—OH, —NH—(CH2CH2)—S—(CH2CH2)—OH, —NH—(CH2CH2)—NH—(CH2CH2)—OH, NH—CH2CH2—CON(CH2CH2OH)2, R3 is —CN or —CONH2, R4 is methyl or trifluoromethyl, R5 is —CF3, —COOC2H5, R6 is hydrogen or chlorine and R7 is hydrogen, chlorine, bromine or —NO2, to a process for the preparation thereof and to the use thereof in a method of producing mass-coloured plastics or polymeric colour particles