How does the Colour Index classify commercial colorants?

The Colour Index (C.I.) classifies commercial colorants—which include both dyes and pigments—based on three primary criteria: chemical structure, method of application, and colour.1

The index is organized into three parts to manage this vast array of information:

• Part 1: Lists colorants by generic name, categorized by application classes (such as acidic, basic, mordant, disperse, or direct) and further subdivided by colour. This section also provides technical data regarding application methods, usage, and fastness properties.1
• Part 2: Details the chemical structures and manufacturing methods for the colorants.1
• Part 3: Provides an index of commercial and generic names, along with the names of the manufacturers.1

While the Colour Index provides a standardized system to navigate the nomenclature of thousands of products, it is important to note that each C.I. generic name represents colorants with the same structure, but not necessarily identical products in terms of particle size, impurity content, or crystal structure. Manufacturers may also assign their own unique trade names to these colorants.1

(Note: The "colour index" in astronomy is a distinct concept used to measure the temperature and brightness of stars and is unrelated to the classification of commercial dyes.)2