Well according to me , there are two posiible reasons for this. One intrinsic and other extrinsic.
The major cause is the amount of melanin secreted by the MSH in the middle lobe of pituitary . Their secretion determines the colour of the skin..
Another reason is the one which is quite common cause of skin darkening and skin tan. Well, that's TAN, ie. The darkening of skin on exposure to sunlight