Horses exhibit a diverse array of coat colors and distinctive markings. A specialized vocabulary has evolved to describe them.
While most horses remain the same coat color throughout life, some undergo gradual color changes as they age. Most white markings are present at birth, and the underlying skin color of a healthy horse does not change. Certain coat colors are also associated with specific breeds, such as the Friesian, which is almost exclusively black.
The basic outline of equine coat color genetics has largely been resolved, and DNA tests to determine the likelihood that a horse will have offspring of a given color have been developed for some colors. Discussion, research, and even controversy continue about some of the details, particularly those surrounding spotting patterns, color sub-shades such as "sooty" or "flaxen", and markings.
Basic coat colors
The two basic pigment colors of horse hairs are pheomelanin ("red"), which produces a reddish brown color, and eumelanin, which produces black. These two hair pigment genes create two base colors: chestnut, which is fully red, and black, which is fully black.
All other coat colors are created by additional genes that modify these two base colors. The most common modifier creates point coloration of both red and black hairs, known as bay, which is classified as a base color as well. The vast range of all other coat colors are created by additional genes' action upon one of these three base colors.
Chestnut/Sorrel: A red coat with no black. The mane and tail are the same shade of chestnut or lighter chestnut than the body coat. The main color variations are: Liver Chestnut: very dark red chestnut coat. Sometimes a liver chestnut is also simply called "brown". Light Chestnut: seldom-used term for a pale chestnut coat, mane, and tail Flaxen Chestnut: Any shade of chestnut, with a significantly lighter mane and tail
Black: Black is relatively uncommon, though it is not "rare". There may be two types of black: fading black and non-fading black. Many, though not all, black horses will fade to a brownish color if the horse is exposed to sunlight regularly. This may be due to nutrition or genetics. Some horses appear to have a non-fading black coat that does not fade in the sun. Most black foals are usually born a mousy grey color resembling grullo. As their foal coat begins to shed out, their black color will show through. For a horse to be considered black, it must be completely black except for white markings. A sun-bleached black horse is still called a black horse, even though it may appear to be a dark bay or brown. A visible difference between a black and a dark chestnut or bay is seen in the fine hairs around the eyes and muzzle. On a black these hairs are black, even if the horse is sun-bleached; on other colors, they will be lighter. The hairs around the coronet band of the hoof will be black on a black horse, but on a chestnut (no matter the shade) they will always be red. Bay: Body color ranges from a light reddish-brown to rich chocolate brown with black points: the mane, tail, lower legs, and tips of the ears. The terminology for various color variations are: Dark Bay: a dark brown or dark reddish-brown coat with black points, difficult to distinguish from seal brown. Sometimes also called "black bay" or "mahogany bay." Blood Bay/Red Bay: a bright red chestnut coat with black points Brown: The word "brown" is used by some breed registries to describe dark bays. Informally, "brown" is applied to many distinct coat colors. Most often, horses described by casual observers as "brown" are actually bay or chestnut.
In the absence of DNA testing, chestnut and bay can be distinguished from each other by looking at the mane, tail and legs for the presence of black points. There is a proposed allele that darkens a bay coat to seal brown, and the sooty gene is linked to other forms of dark bay.
Genetically, a chestnut horse is a horse without the ability to produce black pigment, while a black horse does not have dominant agouti to restrict their black pigment to points. The MC1R (extension) either binds alpha-MSH and signals for black and red pigment to be produced ('E' at extension), or it only signals for red ('e' at extension). ASIP (agouti) either blocks MC1R from binding to alpha-MSH and signaling for black ('A' at agouti), or it does not ('a' at agouti). The extension gene determines whether the cells can decide to produce black and red, and can be either E (able to produce black and red) or e (only able to produce red, as in chestnut). To be chestnut, a horse must have two copies of e, so the genotype is e/e. A horse with a genotype of E/E or E/e can still make black and red pigments and will be bay or black. Meanwhile, the agouti gene determines whether the cells can stop producing black. The A version of agouti means that it can, so as long as has E at extension the base color will be bay. The a version of agouti means the cells cannot stop producing black, so a horse with two copies of a (genotype a/a) and E at extension will be black rather than bay.
Points
The word "points" is given to the mane, tail, lower legs, and ear rims with respect to horse coloration. The overall name given to a horse's coat color depends on the color of both the points and the body. For example, bay horses have a reddish-brown body with black points.
Point coloration is most often produced by the action of the agouti gene. It acts on the extension gene, when present, to suppress black color to all but the extremities of the horse; the legs, mane, tail and tips of the ears. If the extension gene is not present, the effect of agouti is not visible, as there is no black color present to suppress.
Other genes, such as those for white markings, may affect a horse's coat color in addition to agouti, and if present, can further alter or suppress black hair color and may mask any point coloration. In particular, Gray horses are born dark and lighten with age; if born bay, they will eventually lose point coloration as the body hair silvers with age, though often the points are the slowest areas of the body to go gray.
Point coloration may also be visible on horses with other dilution genes that act upon a bay base coat. These include: The cream gene, an incomplete dominant, that when heterozygous produces Buckskin. When homozygous, even point coloration is mostly suppressed, the color is called perlino, and some individuals may have slightly redder hair at the traditional point coloration locations. The Champagne gene, which on a bay base produces Amber Champagne.
A dilution gene that produces what looks like point coloration, but from a completely different genetic mechanism is the dominant Dun gene, which dilutes the color of the body coat but not the points, including primitive markings—a dorsal stripe down the back and, less often, horizontal striping on the upper legs. On a bay base coat, the dun gene leaves black points, producing a Bay Dun or "Zebra" Dun. But the gene also leaves the points dark when it appears with other base colors. These include the “blue dun” or grullo, which has a black base coat, and the red dun, which has a chestnut base coat.
Similarly, darker coloration at the points is also preserved in horses with the roan gene, a patterning gene, producing a body coat of mingled white and dark hairs, but leaving the points the darker base color in all horses, not just those carrying agouti.
Most other genes that produce spotting patterns or white markings allow point coloration produced by agouti to show except where masked by white depigmentation. There are not always separate names for a pattern over a bay base coat, but one exception is the Bay pinto, sometimes called "tricolored".
Gray
A gray horse can be born any color, but as it gets older, some hairs turn white. Most will eventually develop a complete white or a "fleabitten" coat, which retains speckles of the horse's original color. Grays are sometimes confused with certain roan, dun, or white coat colors. In particular, most "white" horses are actually grays with a fully white hair coat. A gray horse is usually distinguishable from a dominant white or a cremello horse by dark skin, particularly noticeable around the eyes, muzzle, flanks, and other areas of thin or no hair. A roan has intermixed light and dark hairs similar to a young gray horse, but unlike a gray does not lighten to white. Dun horses have a solid-colored hair coat that also does not lighten with age. Gray horses are prone to equine melanoma.
Variations of gray that a horse may exhibit over its lifetime include: Steel Grey/Iron Grey: A grey horse with intermingled black and white hairs. This color occurs in a horse born black, or in some cases, dark bay, and slowly lightens as the horse ages. Rose Grey: A grey horse with a reddish or pinkish tinge to its coat. This color occurs in a horse born bay or chestnut and slowly lightens as the horse ages. Dapple Grey: Grey coat with lighter rings of grey hairs, called dapples, scattered throughout. Will eventually fade to a pure white or fleabitten coat. Fleabitten Grey: an otherwise fully white-haired horse with red or dark grey dots flecked throughout the coat, sometimes extensively.
Diluted colors
Several different genetic allelic families produce colors that are lighter versions of the base colors, caused by dilution genes.
Cream family
Cream dilution is an incomplete dominant gene that produces a lightened or "partial dilute" coat color when one copy of the allele is present and a fully dilute (or "double dilute") with two copies. The double cream dilute phenotypes overlap regardless of base coat color and often cannot be distinguished visually. Sometimes the creme allele is combined with an unrelated dilution gene from another family, which creates a cremello-like coat. Such coloration is called a "pseudo-double dilute." These distinctions usually require DNA testing to verify which alleles are present.
Cremello: A horse with a chestnut base coat and two cream genes that wash out almost all color until the horse is a pale cream or light tan color. Often called "white", they are not truly white horses, and they do not carry the white (W) gene. A cremello has blue eyes.
Buckskin: A bay horse with one copy of the cream gene, a dilution gene that "dilutes" or fades the coat color to a yellow, cream, or gold while keeping the black points (mane, tail, legs). They usually have brown eyes. Palomino: chestnut horse that has one cream dilution gene that turns the horse to a golden, yellow, or tan shade with a flaxen or white mane and tail. Often cited as being a color "of twenty-two carat gold", palominos range in shades from extremely light, almost cremello, to deep chocolate, but always with a white or flaxen mane and tail. They usually have brown eyes. Perlino: similar to a cremello, but is genetically a bay base coat with two dilute genes. Eyes are usually blue. Mane, tail and points are not black, but can sometimes be darker than the body coat, generally a reddish or rust color, though not to be confused with a red dun. Smoky black: A horse which visually appears to be only black, but actually has a black base coat and one copy of the cream gene. Smoky Cream: Virtually indistinguishable from a cremello or perlino without DNA testing, a horse with a black base coat and two copies of the cream gene.
Dun
Dun is Yellowish or tan coat with primitive markings, sometimes called "dun factors": a darker-colored mane and tail, a dorsal stripe along the back and occasionally faint horizontal zebralike stripings on the upper legs and a possible transverse stripe across the withers. There are several variations: Grulla, or Blue Dun: A horse with a black base color and the dun gene. Coat is a solid "mouse-colored" gray or silver (can also be almost brownish-gray) with black or dark gray primitive markings.