Lighly complected though they are, the palms of our hands (and soles of our feet) tan slightly when exposed to the sun. But these areas of our bodies never become as dark as our shoulders, for instance, regardless of the amount of light they receive. Cells in the epidermal layer of our skin produce melanin, which colors complexion and protects against UV rays. Increased sun exposure stimulates these cells, called melanocytes, to step up the production of melanin and darken the skin.
The thick layer of dead skin that covers the undersides of feet and hands reduces the amount of UV light that penetrates to the living layers beneath. Rays that make it through find fewer melanocytes to stimulate. And the sparsely populated cells they do find distribute melanin less uniformly, giving the skin a lighter appearance than other areas.