Grey is frequently considered one of the basic colors of horses, but it is more correct to think of it as a pattern of white hairs. Further, it is the only pattern that changes systematically and predictably with age.
The grey locus is well documented, with two alleles. Grey is dominant to wild-type, and is due to a “4.6 kilobase duplication into intron six of the STYX17 (syntaxis 17) locus, on chromosome 25.” The practical meaning of this is that the grey gene can be tested for, and carriers of wild-type identified.
Gray is a pattern of interspersed white hairs that increase in a fairly predictable fashion with age. I say fairly predictable, because there are several patterns of greying, and any genetic controls for which pattern will occur have not yet been found. The speed at which greying occurs is also quite variable, though in most cases a horse is light grey or white by ten years of age. In all cases, however, the greying begins first on the head. This is in sharp contrast to roan, where the horse is born roan and the head remains dark.
Greys can be born almost any color, but when the foal coat is shed, the horse can usually be identified as a grey. Other changes are more variable. The foal may be born with red body pigment, and remain red as the white hairs begin to appear, leading to a rose grey—often miscalled a roan. A red foal coat may shed to black, which then greys as the fraction of white hairs steadily increases. Or the foal may be born black, regardless of the genetic color, and then grey from the black.
Some greys develop a white mane and tail early. These horses generally become pure white with age, though their skin normally remains dark.
Others retain a dark mane and tail as the body lightens. These individuals may retain some dark shading on the legs and even body for a long time, and some never become entirely white.
Some grays are dappled at the intermediate stages—the body is covered with circular areas of lighter hair surrounded by darker circles. Others are more uniform—iron greys. Many, as they grow older, develop reddish flecks and are called flea-bitten greys. So-called blood marks—larger areas of red coat—may also develop.
One down side of grey is that greys are particularly prone to developing melanomas. Usually these are benign, but not in all cases.
It is worth pointing out that all “white” horses with dark skin are actually grey. All other genetic mechanisms for a white coat in horses also produce pink skin.
Greys can have any of the dilution or white marking patterns in addition to the grey pattern. I had a grey and white frame (paint) myself at one point, and while he looked white with slightly darker mane and tail, the frame markings stood out sharply when I bathed him—the skin under the grey areas was black, while that under all of his white markings was pink. He eventually developed a flea-bitten pattern only over the dark skin.
Two greys are mentioned in Homecoming. The first is Derik’s grey, probably a dappled grey. Coryn took the paralyzed Roi for a ride on the second, Cotton.