Why aren't people the same colour?

Colour of people - people vary in the colour of their skin because of a network of pigment forming cells called melanocytes. This network is interspersed between, and lies underneath, the cells of the deepest layer of the epidermis, or outer skin, which is called the stratum basale. The melanocytes have slender, branchlike extensions which touch one another and also extend upwards between the cells of the deeper portions of the epidermis. There are about 1,000 to 3,000 melanocytes in each square millimetre of skin, and each one produces the dark pigment melanin formed as a result of oxidation. This oxidation is catalysed by a copper containing enzyme called tyrosinase, which gives the reddish spectrum of colour changes. Various stages of formation produce is pale yellow, tawny, orange, reddish, brown and, finally, intensely black. 

Human skin contains greater or lesser amounts of melanin. In fair  skinned races the deep skin layer of melanocytes contains very little pigment. In the darker races, the deposits are heavy, and other melanocytes are to be found in the upper layers of the epidermis. Melanin is a natural    protection from harmful sunrays and, on exposure to sunlight, man's skin normally undergoes gradual tanning. This increase of melanin pigment, helps to safeguard underlying tissues. In blondes and redheads the pigment cells respond only slightly and rather unevenly. The consequence  of this may be a "freckling" effect rather than a sun tanned look.