Agree with the above posts, study color genetics a bit more. The only horse I've heard of being prone to melanomas is a grey.
Their skin is black which means a higher melatonin levels than other horses and more chances of skin cancer. Most other coat colors have white skin underneath the coat and less melatonin so they're less likely for melanoma
I don't think so. Most horses skin is black, Pink skin on true white, under white markings and one other color I can't remember, otherwise skin is black. I don't know why greys are more prone to melanomas, very curious to find out.