I bred and showed Siamese cats for years - they have blue eyes but not completely white coloring; still they do seem to have a number of genetic problems that are peculiar to Siamese. The gene that produces the color in Siamese creates a pigment that is temperature sensitive, which is why their extremities (which are cooler) are darker than everything else, and their tummies and chests are lighter than their backs. One of my cats likes to nap with her head over one of the floor registers - her mask has turned a mottled sort of tortie color from the constant heat.
I did not know they were temperature sensitive. I have a feral cat (one of four) that has the markings of a siamese or burmese except that her nose is mottled with calico. Her back darkens during the winter and then lightens up during the summer. I thought she'd just been around appaloosas too long.
It's funny that you mention that coz I've wondered if the same thing happens in roan horses that have dark points like mine. They sort of remind me of Siamese cats. I've even called 'em Siamese horses before. They have such a marked color change between summer and winter and I wonder if it's temperature related. Bob is almost white in the body in the summer and almost black in the winter. It's weird.