Process for preparing pigments of the formula (I) or tautomeric structures thereof and their hydrates, containing melamine as guest, characterized in that a mixture of the azobarbituric mono-cation salt (II) and the azobarbituric di-cation salt (III) is reacted with a nickel compound and melamine, in which cation1, cation2 and cation3 independently of one another are any desired univalent cations or the fraction of any desired multivalent cation that corresponds to a positive charge of one.