A process for mass colouring high molecular weight organic material by using a pyrimidopteridine of formula (I) and/or (II) or a mixture thereof, wherein A, B, C and D are each independently of one another -NH2, -OH, hydrogen, C1-C4-alkyl; phenyl, biphenyl or naphthyl which are unsubstituted or substituted by halogen, -OH, -NH2, C1-C4alkyl or C1-C4alkoxy; -NHR1, -N(R1)2 or formula (1), wherein R1 is C1-C4alkyl; phenyl which is unsubstituted or substituted by halogen, C1-C4alkyl or C1-C4alkoxy, or -COXR2, X is a direct bond, -O- or -NH-, and R2 is C1-C4alkyl; phenyl which is unsubstituted or substituted by halogen, C1-C4alkyl or C1-C4alkoxy, with the proviso that at least two of the radicals A, B, C and D are -NH2 or -OH.