We had a solid chestnut mare at the barn two summers ago who is a papered morgan with no outbreeding. She is chestnut in the winter, and developed random white spots all over her body two summers ago. Big ones, ALL OVER (not just muzzle/eyes, though she had some there as well). The vet suspected a mineral deficiency (cannot recall what the deficiency was) and started the mare on a supplement. The white spots disappeared and did not come back this summer.