The present invention relates to a method of coloring keratin-containing fibres, especially human hair, in which method a compound of formula (I), wherein R is hydroxy or NHR.sub.1, R.sub.1 being hydrogen or C.sub.1 C.sub.4alkyl, and B is an aromatic or heterocyclic ring, is diazotised, and the resulting diazonium salt is applied at a pH of from 2 to 5 to the material being colored and then, by means of increasing the pH to from 7 to 12, is fixed on the material in the form of an oligomeric or polymeric colorant ##STR00001##