Gray horses are prone to vitiligo, which is the gradual and progressive dipigmentation of the skin. People get it, too.
Homozygous grays are more likely to end up with a significant degree of vitiligo, giving the skin a mottled appearance, but heterozygous grays certainly get it, too. There appears to be no link between vitiligo and melanoma.
As for the color of a gray's skin, it will come from the base color of the horse--so an Appaloosa gray will likely have pink or mottled skin, a black gray will have dark skin, a cremello gray will have pink skin, and so on. In the general scheme of things, the majority of grays will have dark skin simply because the majority of horses have dark skin.
The cancer that grays get is melanoma, most typically, whereas white/spotted/pink-skinned horses are more prone to squamous cell carcinoma.
I don't know what those are on your horse, Phantom. They do remind me of smrobs' horses melanoma, but at the same time, they also resemble sarcoids.