The color of your cat's hair, skin and eyes are all determined by melanin. Chemically speaking, there are two kinds of melanin: eumelanin and phaeomelanin. The former absorbs almost all light, which makes it appear black as fur and gray as skin. The latter reflects light in the red to yellow color range as fur and pink as skin. The density of melanin can vary from one part of the body to another, down to patterns within individual hairs or gradients across skin, thus accounting for a wide variety of cat colors. Cats' skin appears strongly tied to fur pigmentation. As such, it's hard to talk about one without referencing the other.