The Colour Index (CI) is a list of identification numbers and names given to individual pigments and dyes used in, among other things, art materials. It is published by the Society of Dyers and Colourists (UK) and Association of Textile Chemists and Colorists (USA).
An entry in the Colour Index has three parts:
The Pigment Codes used the Colour Index:
So, for example, the colour a painter generally calls French ultramarine has the Colour Index Name and Number PB 29 (77007), so contains Pigment Blue 29 or coluor 77007.
Colour Index information is particularly useful when it comes to identifying what is in a particular tube of paint (quality artist's paint will include the pigment number(s) on the label). It'll also show you whether a particular paint colour is made from a single pigment or a mixture, which is especially useful information when colour mixing.