The Color Index (CI) is a list of identification numbers and names given to individuals 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).
A entry in the Color Index has three parts:
- Use (whether it's a pigment, or dye).
- Color (indicated by a pigment code and number).
- Constitution Number (a unique number, allocated sequentially as a substance is added to the Color Index).
The Pigment Codes used are:
- PB = Pigment Blue
- PBk = Pigment Black
- PBr = Pigment Brown
- PG = Pigment Green
- PM = Pigment Metal
- PO = Pigment Orange
- PR = Pigment Red
- PV = Pigment Violet
- PW = Pigment White
- PY = Pigment Yellow
So, for example, the color a painter generally calls French ultramarine has the Color Index Name and Number PB 29 (77007), so contains Pigment Blue 29 or color 77007.
Color 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 color is made from a single pigment or a mixture, which is especially useful information when color mixing. (See Identifying What's in a Paint Tube.)