Sodium nitrite, which contains nitrogen, is a convenient source of nitrous acid in the manufacturing or dyes and pigments.  When primary aromatic amines react with nitrous acid, the intermediate diamine salts are produced which, on coupling to amines, phenols, naphthols, and other compounds form the important azo dye.  The color center of the dye or pigment is the  N=N  group and attached groups modify the color.  Many dyes and pigments have been manufactured with shades of the entire color spectrum.