Somatic mutations or mismarks are non genetic markings that cannot be passed on to offspring. Somatic mutations are due to conditions in the womb or genetic "mistakes" and cannot be bred for.
Many somatic markings are simply "mismarks". Random spots, often white but sometimes dark. Another type are color "glitches" and a part of the horse is a different color. For example, a large grey patch, while the horse isn't a grey. Another type has big white grease spots.