A colourant is a substance that is used to impart colour to matter. Dyes and pigments are the main forms of colourant. The main difference between them is that dyes are soluble and pigments are insoluble and are suspended in a medium or binder. This is due to the difference in particle size of dyes and pigments which affects the way they behave.
Dye particles are much smaller than pigment particles – imagine the difference between a pinhead (dye) to a football (pigment). As dyes are soluble and pigments insoluble, you might then imagine dye particles dissolving in water, while pigment particles must be suspended in a binder – imagine the comparison of salt in water (dye) or rocks in water (pigment). The salt will dissolve in the water, creating a solution, while the rocks will sink to the bottom creating a suspension.
Another difference is bonding properties, so where a dye might chemically attach itself to a substrate on a molecular level, becoming part of the material, pigments require the binder or carrier to act as a glue of sorts that is painted on to the substrate and surrounds the pigment and keeping it in place. So dyes become part of the material and pigments sit on top in a layer. These rules are general, so sometimes, depending on the material or dye, a dye will need a mordant to help it bind.
One of the behavioural differences is their lightfastness properties (the level at which they fade when exposed to light). Pigments have the ability to resist this fading process, whereas dyes are more vulnerable to fading or bleaching caused by ultraviolet light from the sun.
Sunlight (UV rays) can break electronic bonding of a dye molecule and destroy its colour, which is why dyes fade; imagine a pair of denim jeans fading or a patch of wallpaper where there had been a hanging picture. Some people will choose dyes purposely for this reason and enjoy the natural fading process, while others will require a more permanent colour.
While most pigments are lightfast, there are also a few pigments called “fugitive pigments”, such as Rose Madder, that will fade over time. Again, depending on your intent for the colourant, you may prefer one that fades naturally.
Blue Wool Scale
The Blue Wool Scale was created to measure these levels of lightfastness. Strips of colourant are placed in laboratory ultraviolet conditions designed to mimic sunlight over many years. Part of the strip is protected from the light and after a period of exposure time the strip is revealed and the fading can be measured on the Blue Wool Scale from 1-8, 8 being the most fade resistant. This test was designed for the textiles industry, and has now been adapted to test pigments as well.