A process for preparing a black colorant, preferably a black pigment, characterized in that a compound of the Formula (I) is reacted with a compound of the Formula (II) in a molar ratio of 1:2, in the presence of a catalyst which in water at 25° C. has a pK≦4.5. For the definition of R1 to R7, refer to the description; preferably all are H. Likewise claimed are new crystal polymorphs obtainable by this process, and preferably single-phase mixed crystals. Also claimed are synergistic compositions. These compositions exhibit increased light stability in plastics stabilized with basic light stabilizers when they are pigmented with colorants of the invention rather than with known black pigments such as carbon black. The specification also relates, furthermore, to the use of the black colorants of the invention for coloring paints, printing inks and plastics in the mass, and to mulch film, instrument panels, woven fabrics, garden furniture items or elements for the construction industry that are pigmented with said colorants.