Horse coat colour is defined by pigment called melanin. One form, eumelanin, gives black pigment as in black horses, while another, pheomelanin, gives red pigment as in chestnuts. All horse coat colours are based on these 2 pigments.

BASIC COLOURS.

There are 4 basic coat colours, black, brown, bay and chestnut.
 

 

Black horses appear completely black all over, although some may show a slightly rusty tinge if the coat has faded. Black foals usually have silver grey legs in their birth coat. Black as a colour is dominant to chestnut, but is modified by bay or brown.
 

  Brown  horses look very nearly black. Browns may have a tan muzzle, tan around the eyes, in front of the stifles and behind the elbows.

  Bay  is basically black modified by the agouti or baying gene. Agouti confines black to the points so bay horses have black mane, tail and lower legs while the body, neck and head may vary from a light sandy colour through a dark mahogany to a near brown colour. Some bays show little black on their legs and have pale hair in the mane and tail so could be confused with dark chestnuts, but a bay horse always has black ear rims. This often helps to decide the basic colour of a bay tobiano. A bay foal may closely resemble a chestnut at birth as it has silver grey legs like black foals. Bay is dominant, so cannot be carried by black, but it can be carried unseen by chestnut, which explains why a black crossed with a chestnut sometimes produces bay.

  Chestnut  comes in a very wide variety of shades. It varies from a pale sandy orange colour through red to dark liver. The mane and tail may be considerably lighter, as in flaxen, the same colour as the body or much darker. Chestnut is recessive to black, which means a black or bay horse may carry chestnut without showing it. Both parents must carry chestnut in order to produce a chestnut. As chestnut cannot carry black, 2 chestnuts will always throw chestnut. The exception to this is the unusual mushroom colour that appears in some blood lines in UK registered Shetlands.
    All other shades, dilutes and patterns are built on these 4 basic colours. They may be inherited singly or in multiples to give some very complex 'colours' that are sometimes hard to identify.