A horse's coat color that mixes white and black hairs is called gray. In horse terminology, “gray” describes a coat affected by progressive depigmentation, rather than a simple blend like gray paint.
Many gray horses are born dark—black, bay, or chestnut—and gradually develop more white hairs as they age. Some become almost completely white, but they are still classified as gray. The gray gene can occur in many breeds, including Arabians, Thoroughbreds, Andalusians, Lipizzaners, and American Quarter Horses.
Gray is commonly confused with roan. Roan horses have white hairs permanently mixed with colored hairs, while gray horses usually become lighter over time. A gray horse that appears white often still has dark skin and dark eyes; a genetically white horse generally has pink skin.
Gray horses can also develop “flea-bitten” patterns, in which small red or dark speckles appear on an otherwise pale coat. Gray horses are notably prone to equine melanoma, a type of skin tumor.