Despite all the varieties of colours and patterns visible on dogs, there are only two pigments – eumelanin (black) and pheomelanin (red). All coat colours are made up of these two pigments, two forms of melanin. Each of the pigments has a basic colour which may be modified by different genes.
Eumelanin is the basic type for black pigment. Eumelanin producing cells cause black areas on a dog. However, there are genes making eumelanin to other colours such as brown, blue or isabella (a moderate yellowish brown). If a dog has any of the genes that makes the black pigment change to brown, blue or isabella, the entire black coat will change. The reason is that these genes inhibit or change the production of eumelanin so that none of the cells may produce a fully black pigment. Dogs with the diluted colour are called blue or isabella. Technically the brown colour is not diluted but is caused by a change in the structure of the pigment. Eumelanin is present on other body parts of the dog such as the nose and eyes.
The other type of pigment is pheomelanin which is the red pigment. Pheomelanin is only produced in the fur and does not occur in the eyes or on the nose. In contrast to eumelanin it does not come in two colours. Pheomelanin is just one colour which varies in intensity. There are variations from a dark red in dogs as the Irish Setter to pale yellow as in a very light-coloured Labrador. Most dogs, however, have both eumelanin and pheomelanin. A black and tan Kelpie is a wonderful example of this.
The melanin pigment is produced in the melanocyte cells. These are pigment cells located in the neural crest found already in the embryotic stage. The pigment cells migrate to the skin, either covering parts of the body or the entire body, and these cells has a lifelong production of the pigment melanin. Pigment cells and nerve cells have the same origin. This means that genes controlling pigment cells also control neurological and immunological functions. Hence the connection between specific colours and illnesses in some dog breeds.