A method for changing the colour of oxynitrides contg. 2-3 different cations (Q, R, S), at least two of which have a different valency between +1 and +6, during their prepn. by heating under nitride-forming conditions until red hot a powdered mixt. contg. oxynitride-forming metal cpds., comprises shifting the long-wave oxynitride absorption limit whilst keeping the crystal structure unchanged by increasing the N to O atom ratio by substituting n equivs. O with n equivs. N, and at the same time substituting n equivs. Q (valency = q) by n equivs. R (valency = q+1), whereby n is a no. between zero and the no of Q atoms in the structure in the absence of R, whilst q is a whole no. from 1-5. The process is reversed by a short-wave shift in the absorption limit. Dye pigments prepd. by this method and contg. pyrochlore crystal oxynitrides of formula (Ia) or (Ib), spinel crystal oxynitrides of formula (IIa) or (IIb), elpasolite crystal oxynitrides of formula (IIIa), (IIIb) or (IIIc), or perovskite crystal oxynitrides of formula (IV) or (V) are also claimed; AxA'2-xB2O5+xN2-x (Ia), A'2B2-yB'yO5+yN2-y (Ib), CD2-mD'mO4-mNm (IIa), C1-nC'nD2O4-nNn (IIb), A'2CBO5N (IIIa), A'2A"BO4N2 (IIIb), A'2DBO3N3 (IIIc). A = Mg<2+>, Ca<2+>, Sr<2+>, Ba<2+> or Zn<2+>; A' = Ln<3+> (rare earth element), Bi<3+>, Al<3+> or Fe<3+>; A" = Ln<3+> or Bi<3+>; B = V<5+>, Nb<5+>, Ti<5+>, Mo<5+> or W<5+>; B', D' = Ti<4+>, Zr<4+>, Hf<4+>, Sn<4+>, Ge<4+>, Si<4+>, Nb<4+> or Ta<4+>; C = Mg<2+>, Ca<2+>, Mn<2+>, Fe<2+>, Co<2+>, Ni<2+> or Zn<2+>; C', D = Al<3+>, Ga<3+>, In<3+>, Ti<3+>, V<3+>, Cr<3+>, Fe<3+>, Co<3+> or Ni<3+>; x, m = 0-2; and n = 0-1 with the provision that (Ia) or (Ib) is not Ln2Ta2O5N2. A1-uA'uBO2-uN1+u (IV), A'B1-wB'wO1+wN2-w (V), A = Mg<2+>, Ca<2+>, Sr<2+> or Ba<2+>; A' = Ln<3+>, Bi<3+>, Al<3+> or Fe<3+>; B = V<5+>, Nb<5+> or Ta<5+>; B' = Ti<4+>, Zr<4+>, Hf<4+>, Sn<4+> or Ge<4+>; and u, w = 0-1, with the provision that when A' is Ln then u is not 1 and w is not zero.