Skin color is determined by the amount of melanin produced by melanocytes and deposited in basal cells.
There are several skin disorders of pigmentation. A good source of information on skin pigmentation disorders can be found here at Medline Plus (scroll down to specific disorders).
Here are some other specific disorders of pigmentation.
Freckles and moles
Some other diseases of the epidermis are:
Psoriasis Photo's here.
Seborrheic dermatitis Photos here (scroll down)
Lichen simplex chronicus