Faceman, I don't know that I've ever read any statistics about the rate of grays actually dying from melanoma, but it's common enough to be disconcerting. I knew a mare who had it internally, causing horribly diarrhea and weight loss until she had to be euthed. A friend's horse had it so that it cut off her airway, again leading to euthanasia. I've heard of it strangulating the bowel and causing fatal colic, or causing horrific (thus fatal) neurological damage by butting up against the spinal cord. There is a mare in my vet's practice--if she hasn't died yet--who has them all over her body, just subdermal. There's a football-sized lump at her throat. She had to have a permanent tracheotomy. It's horrible. Some of these horses, oddly enough, never displayed the outward signs of lumps and bumps before dying of internal cancer. I really don't know how it works, or what causes it to be harmless in one horse and deadly in another. All I know is that in terms of getting it in the first place, homozygous grays are at an increased risk, as are horses in general who do not possess a copy of the A agouti gene.
With pink-skinned horses, I always thought the dominant cancer to watch out for was squamous cell carcinoma? I may be wrong, but I've seen several cases of that on the eyes and genitals of some Appaloosas I know.
Oh, and Faceman, I believe they've all but given up on using gray horses as models of human melanoma, unless they've recently found some new application. The inheritance and expression of the cancer is quite different between the species, which is not what they'd hoped to find.
LadyDreamer, I believe those are dominant whites.