Created in 1925, the Colour Index International is a database of pigments and dyes published by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. Colourants are given many different proprietary and generic names, but the Colour Index International provides a standardised system used by manufacturers and consumers all over the world that can be a hugely informative tool for artists.
What is a Pigment?
Pigments are fine coloured powders that can be mixed with a binder – such as linseed oil, gum arabic, or acrylic polymer – to make artist paints. While dyes are soluble and dissolve in the binding medium (enabling them to chemically bind with a material – ideal for colouring textiles), pigments are insoluble and are suspended within the binding medium. The manufacture of artist paints consumes a tiny fraction of the pigment industry – the vast majority of pigments are used in larger industries, like in the production of cars and plastics.
How Does the Colour Index Work?
Unlike other colour systems, like Pantone or the Munsell System, the Colour Index International groups dyes and pigments according to their chemical composition rather than the exact hue, value, and chroma of the colour they produce.
Each pigment entry has two identifying codes – the Colour Index Constitution Number, and the Colour Index Generic Name Code. For example, Ultramarine Blue is categorised under CI 77007 (the Colour Index Constitution Number) and PB 29 (the Colour Index Generic Name Code). The Colour Index Generic Name Code is most recognisable to artists, and it’s common to find one or more of these codes listed as part of the colour description before you choose it, and on the tube or bottle of paint:
The letters classify the colour group, e.g. PB= Pigment Blue, PW= Pigment White, PV= Pigment Violet, etc. The number classifies the chemical composition – so PB 29 can be read as ‘Pigment Blue 29’ – identifying it as sodium-aluminium-sulpho-silicate, or Ultramarine Blue (like on the Williamsburg tube above, some manufacturers add the chemical formula alongside the Colour Index Name Code, but not all do). The number is not related directly to any part of the actual chemical structure, rather they are assigned chronologically in order of when the pigment was added to the index (not when the pigment was discovered).
Paint manufacturers are not obliged to disclose what pigments are in their paints. Some pigment mixtures, like Winsor & Newton’s Cadmium-free colours, are proprietary and the Colour Index Codes aren’t given. Most paint ranges, particularly professional ones, do include them on the label.
Why is the Pigment Colour Index Useful for Artists?
Here are some of the reasons why it can be so useful to become familiar with the Pigment Colour Index:
1. The Pigment Colour Index provides insight into how different paints behave
All pigments have individual characteristics including transparency/opacity, tinting strength, granulation (in watercolour), drying rate, lightfastness, and permanence – all of which huge impact on your painting. These pigment properties are determined by their physical attributes and chemical structure, and while the exact science behind this is complex, knowing how certain pigments behave is a great way to get the most from your artist paints.
2. Colour names are not always reliable
You can’t always rely on the name of the colour. As an example, here are the pigments used in a selection of paints labelled ‘Naples Yellow’, a historical toxic pigment that is often recreated for the modern artist using a mixture of pigments. Similar to food ingredients labels, pigments are usually listed in order of predominance (the pigment used in the greatest amount first, followed in descending order by those in smaller amounts).
PW 6 (Titanium White), PY 42 (Synthetic Yellow Iron Oxide) – Old Holland New Masters Acrylic Paint
PW 4 (Zinc White), PW 6 (Titanium White) , PY 35 (Cadmium Yellow) – Sennelier Watercolour Paint
PW 4 (Zinc White), PY 35 (Cadmium Yellow), PR 101 (Synthetic Red Iron Oxide) – Daniel Smith Watercolour Paint
Pbr 24 (Chrome Antimony Titanate) – Michael Harding Oil Paint
PBr 24 (Chrome Antimony Titanate), Pbk 7 (Natural Brown Iron Oxide), PW 4(Zinc White) – Golden QOR Watercolour Paint
PW 6 (Titanium White), PBr 24 (Chrome Antimony Titanate) – Winsor & Newton Watercolour Paint
It’s worth noting that these pigment combinations make a great guide to mixing your own ‘Naples Yellow’.
3. The same pigment can have different names
There can be huge variations in colour names among manufacturers- even for the most commonly used colours. The blue pigment Copper Phthalocyanine was first sold under the trade name Monastral Blue, but is also known by many other names and spelling variations, including Monestial Blue, Helio Blue, Phthalo Blue, Thalo Blue, Winsor Blue, and Scheveningen Blue. The Colour Index International classifies Copper Phthalocyanine as simply Pigment Blue 15 (PB 15). Seeing this on the side of the paint tube, you can quite reliably predict that the paint is transparent, deep in masstone, and very strong in mixtures – traits that are typical in Phthalocyanine pigments.
4. You can identify genuine pigments
Genuine pigments are those which have some historical significance or particular characteristics, but are sometimes recreated using mixtures of other pigments. Viridian Green (PG 18), for example, was used by the Impressionists, but is sometimes recreated using Phthalocyanine Green pigment (PG 7). Another example is genuine Van Dyke Brown (PBr 8), a historical deep brown pigment made from iron oxide, clay, and bitumen, which is usually mimicked with a mixture of pigments. It’s important to note here that it’s easy to attach a certain romance to historical pigments, but they aren’t always the best. There are many toxic or non-lightfast pigments that have now been replaced with safer and more permanent modern pigments.
Top: Genuine Alizarin Crimson (non-lightfast pigment PR 83) mixed with Ultramarine Blue. Bottom: Permanent Alizarin Crimson made with lightfast pigments PR 177, PV 19, and PR 149 in the same mixtures with Ultrmarine Blue.
5. The Pigment Colour Index tells you how many pigments are in the paint
The number of pigments in a paint matters enormously to many artists. Single pigment colours – paints that contain only one pigment – are generally considered to be best in mixtures because the more different pigments are mixed together, the muddier the resulting colour. If single pigment colours are important to you, then the Pigment Colour Index is invaluable to make sure you are using a single-pigment version of a colour.
Why Do Different Colours Have the Same Colour Index Code?
All pigments vary in hue, chroma, and handling characteristics depending on the way they are manufactured and processed, but some entries in the Colour Index are more precise than others. Paints made using single pigment PY 3 (a pigment commonly known as Hansa Yellow Light), are almost always a bright, semi-transparent, lemon yellow. However, some Colour Index Codes cover a wide range of colours with very different characteristics. A good example of this is PBr 7 (Pigment Brown 7). It refers to a Natural Brown Iron Oxide, and due to the natural variations of iron oxides, paints labelled PBr 7 can vary from yellowish-brown to dull red. Pigment manufacturers can control these variations – Burnt Sienna is produced by heating Raw Sienna, which partially dehydrates the iron oxide, making it darker and redder. The same goes for Raw Umber and Burnt Umber, two other Brown Iron Oxide pigments. So, Raw Sienna, Burnt Sienna, Raw Umber, and Burnt Umber can be categorised under PBr 7, despite the fact that they are very different colours.
Left to Right: Burnt Umber, Raw Umber, Raw Sienna, and Burnt Sienna.
Artists instinctively build knowledge about the properties of colour as they are working. By familiarising yourself with the Colour Index you can identify exactly which pigments are best for your practice.