Modifiers change the appearance of the coat color. They are similar to dilutions but have different effects.
The most common modifier in the domestic horse population. It restricts black pigment to the points, leaving the body red in color but the points black. Agouti does not affect chestnut horses but can be carried by them and passed on to their offspring. It is theorized that there are several different alleles of agouti, including:
Classic Bay (A)
Classic Bay is a horse with red-brown body color and black points.
Wild Bay (A+)
A wild bay is almost the same as a classic bay, but the points on a wild bay are noticeably shorter, usually ending around the ankles. They may even be absent altogether. The body color is usually lighter than a classic bay and the mane and tail may be prone to bleaching or mixed with lighter hairs.
Seal Bay (At)
Seal Bays or Browns are very dark and generally have a black, or nearly black, body with red or tan around the nose, eyes, and flanks. Seal bay is controversial, with some arguing that it is actually bay + sooty.
A dominant modifier that changes the coat color to white over the years. It can affect any color or color combination. Some horses will grey more quickly or slowly than others.
Grey horses are usually born with a fully colored coat, like an adult horse; i.e. a bay foal would be born with black legs rather than pale legs that come in black after the foal shed. This is not, however, a definitive indicator. Other signs that experienced breeders look for are chestnut foals born with dark instead of pink-hued skin, and telltale white hairs – even one – in the eyelashes or around the eyes. Greying often begins on the face, creating “grey goggles” around the eyes of foals. But this is also not a definitive sign, and horses can begin to grey on almost any part of the body. Some horses, particularly very dark or black horses, have been observed to begin greying as late as nine years old, while some foals are born having already begun the greying process. Grey horses will have dark eyes and skin (if another dilution, modifier or white pattern has not changed them).
Sooty – (sometimes called smutty)
A modifier that causes black hairs to be mixed into the coat. Generally, the effect is as if someone had dumped a bucket of soot over the horses head and back. This acts as a form of countershading. The dark pigment can also be distributed evenly throughout the coat or concentrated on the mane, tail, legs, or other parts of the body. It may express itself as smudges, patches, striping, dappling, or other primitive markings. It is believed to create dark bays, chestnuts, palominos, and buckskins, but there is debate over whether it causes liver or black chestnuts, and seal bays or browns. Sooty tends to vary seasonally and is susceptible to bleaching and the general condition of the coat caused by grooming, stabling, and nutrition. It is not known at this time what causes sooty.
Pangaré – (also called mealy)
A modifier that lightens the coat of chestnut and bay horses along the flanks, belly, inner legs, at the muzzle, and around the eyes. It often presents as flaxen in chestnut horses. It is believed to not express on black horses. The inheritance of pangaré is not understood.
A modifier that lightens hairs in the mane and tail of chestnut horses. It may also affect the lower legs. It is sometimes confused with palomino or silver. The inheritance of flaxen is not understood.