Colorants may be classified usefully in two separate ways, either according to their chemical structure or according to the method of application. The most important reference work dealing with the classification of dyes and pigments is the Colour Index, a publication produced by the Society of Dyers and Colourists, Bradford, England. This series of volumes provides a comprehensive listing of the known commercial dyes and pigments and is updated on a regular basis. Each colorant is given a C. I. Generic Name, which incorporates its application class, the hue and a number, which simply reflects the chronological order in which the colorants were introduced commercially. It is extremely useful that this system of nomenclature for dyes and pigments is more or less universally accepted by all those involved in their manufacture and application, and so it is frequently used throughout this book. The Colour Index provides useful information for each dye and pigment on the methods of application and on the range of fastness properties. The volumes also list the companies that manufacture each of the products, together with trade names, and give the appropriate chemical constitutions where these have been disclosed by the manufacturer.

In the chemical classification method, colorants are grouped according to certain common chemical structural features. The most important organic dyes and pigments, in roughly decreasing order of importance, belong to the azo ( N=N ), carbonyl (C = O) (including anthraquinones), phthalocyanine, arylcarbonium ion (including triphenylmethines), sulfur, polymethine and nitro chemical classes.