Cats don't shave. As such, it's rare you get a look at their skin. That skin is colored by the same chemicals that color their hair, albeit in less vibrant hues. Odds are your cat's skin color is a lighter version of whatever color or colors his hair is.
Gray and Pink
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.
White, Light and Tawny
Melanin doesn't appear to account for cats' light pink- to white-colored skin. That's because it doesn't. White, at least as far as cats are concerned, is the absence of pigmentation. White fur doesn't necessarily signify total albinism: There are several gene combinations that result in partial, lightly colored coats and skin patterns. Sometimes this is hard to see -- even if your cat was recently shaved. That's because even short hairs reflect light, giving your cat's skin a shading close to that of his coat. Wild cats usually have tawny skin, often between dark and light skin. Domestic cats aren't as regular, in part, because of aggressive breeding programs to refine certain traits and breeds.
Girls vs. Boys
A cat who has darker fur probably has darker skin and vice versa on account of the melanin content of his skin and fur. Tortoiseshell cats and their white-stricken siblings, calico cats, have patches of both dark and light skin. That's because the gene that determines their skin and coat colors carries on the X chromosome. Female cats have two, so they can get both sets of genes. Male cats have one, so they're stuck with whatever version they get. In rare instances -- most often an XXY sterile male -- male cats will have both dark and light fur and skin.
Changes and Other Considerations
Sometimes a cat's skin and hair will change color. It can be age-related, akin to the lightening of skin and graying of hair in humans. It can also be related to trauma, like a bruise, a rash or allergic reaction of some sort. Finally, it can also be related to skin cancer or other diseases. As such it's important to see a vet any time your cat's coloring shifts -- whether it be gradual or sudden -- to make sure nothing's amiss. It might be a false alarm, but it might also save your cat's life.
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