For most horse breeders, coat color is merely a cosmetic aspect, but recently certain colors have been sought after and specifically bred for. Coat color is the result of 11 independent processes with the interaction of the 11 processes causing the end shades and types. The phenotype refers to what you see on the outside of the horse while the genotype refers to what is hidden. White is not a color, but actually a non-color and lacks pigment. The primary hard colors in horses are black, or brown, and chestnut.
Bays are the most common and considered to be the most dominant.
The Arabian breed contains a certain type of dark brown bay that often masks the black gene. Chestnut is the next common and is some shade of red with non-black points. The rarest of the rare is the dominant, or homozygous, black. The genetic control of these hard colors lies at two loci, the Agouti and the Extension . The Agouti determines the color of horses with black points and blacks, and the Extension, the basis for chestnut.