Freckles develop on any skin exposed to the sun. They are clusters of concentrated melaninized cells which are most easily visible on people with a fair complexion.

On different people, freckles may vary somewhat in color. They may be reddish, yellow, light brown, brown, or black but they are basically slightly darker than the surrounding skin.

The more melanin you have in your skin, the darker your skin color! People with fair skin have less melanin in their skin but some of their melanocytes make more melanin when exposed to the sun. So instead of easily getting an even suntan, they sometimes get freckles.

A freckle is essentially nothing more than an unusually heavy deposit of melanin at one spot in the skin.

More Info: en.wikipedia.org