References

Big Site of Amazing Facts; What Gives Your Skin Its Colour?
http://www.bigsiteofamazingfacts.com/what-gives-your-skin-its-color

Costin, G. & Hearing, V. 2007. Human skin pigmentation: melanocytes modulate skin color in response to stress. The FASEB Journal. 21: 976-994.

Jablonski, N. 2004. The evolution of human skin and skin color. Annual Review of Anthropology.        33:585-623

Jablonski, N. & Chaplin, B. 2000. The evolution of human skin coloration. Journal of Human Evolution. 39: 57-106.

Kids Health; Your Skin
http://kidshealth.org/kid/htbw/skin.html#

Medical News Today; What is Vitamin D?
http://www.medicalnewstoday.com/articles/161618.php

News Medical; Melanin- What is Melanin?
http://www.news-medical.net/health/Melanin-What-is-Melanin.aspx

Relethford, J.H. 2002. Apportionment of global human genetic diversity based on craneometrics and skin color. Am. J. Phys. Anthropol. 118, 393−398.

Smithsonian National Museum of Natural History; Skin Colour
http://humanorigins.si.edu/evidence/genetics/skin-color/modern-human-diversity-skin-color

UV Radiation: Pros & Cons

Vitamin D Synthesis: exposure to UVB initiates vitamin D synthesis. Vitamin D is a fat-soluble steroid that is necessary for calcium and phosphorus absorption. It is synthesized in adequate amounts by exposure to sunlight, so taking dietary pills is not necessary. It is also an immune regulator, and has been shown to reduce the risks of rheumatoid arthritis in women.

Medical Use: ultraviolet radiation is used to help treat psoriasis. This treatment helps clear the disorder, however it greatly increases the risk of skin cancer.

Skin Cancer: overexposure to UVB causes sunburns, but also skin cancer. UVC rays are the most dangerous type of ultraviolet light because they are the highest in energy. UVA, UVB, and UVC all directly damage DNA, so protection from all three forms is necessary.

UV radiation walks a thin line between being a benefit and a harm to humans. It is necessary for vitamin D synthesis, however it can be lethal by causing different forms of cancers. Without vitamin D, we can not absorb calcium which is crucial for our bones. However, spending too much time in the sun unprotected can result in cancer.

What Gives Skin Its Colour?

Melanin is responsible for giving skin its colour. The majority of skin cells are busy replacing the epidermis, but approximately 5% are making the brown/yellow pigment melanin. It is produced by melanocytes in the epidermis. The amount of pigment you have in your skin depends on the amount your parents had. Pigments get passed on from generation to generation. People with darker skin express the melanin-producing genes more frequently than lighter-skinned people.
Albinism is defined by having little or no melanin in the body. People that suffer from this condition have extremely pale skin, light hair and eyes. Melanin effects the colour of hair and the iris of the eyes, so they are affected as well.  
When you go out into the sun, more melanin cells are produced to help protect the body from harsh UV rays. As more and more melanin is expressed, the skin becomes darker. That is why skin gets darker as you spend more time in the sun. The tan fades because new skin cells are produced which haven't been as exposed to the sun.

Seafood Affecting Skin Colour?

Seafood is an interesting factor that also surprisingly affects skin colour. Seafood is an excellent source of vitamin D, so people who have diets rich in seafood are high in vitamin D. This is most commonly seen in people who live in coastal areas, where fish is a primary source of food in their diet. A common case for Eskimos, and people who live in the Arctic regions. Since such high amounts of vitamin D are consumed, the skin doesn't need to reduce pigmentation. A pigmentation reduction would be necessary for those who were not eating a shellfish abundant diet. If these people reduce the amount of seafood they are accustomed to eating, they face the risk of vitamin D deficiency which can lead to osteoporosis.

The Blending of Skin Colours

The distinction between such drastic skin colours was more obvious years ago. Without automobiles, planes, and trains, people didn't move around as much. They mated with people from the same areas passing their traits down from generation to generation. Since they were isolated, most of the skin colours were similar.
As technology increased and travelling became accessible people started mating all over the world. Someone that once lived in Africa could move to the north and mate with a light-skinned individual. Skin pigmentation is passed on as a blend, where a number of different shades could result. When two ethnically diverse individuals have children, the children may not have the exact same coloured skin.

The Effects of the Equator

Variations in human skin colour are due largely to geography and the sun's ultraviolet radiation (UV rays). Depending on where you live in the world, there is usually a common pattern in the colours of skin.  Those that live closest to the equator will have a higher percentage of people with darker skin. Those that live at the poles will have drastically lighter skin. Basically, there is a high occurrence of  dark skin in the tropical areas, and light skin in temperate. The reason behind this is that darker skin is required to avoid vitamin D toxicity. While in equatorial areas, the sun is much stronger and dangerous. As people started moving around the world, natural selection created different shades of skin. Those that continued to be in open areas retained their darker skin for protection. However, people that were in shaded protected areas, or places with less frequent exposure to sunlight developed lighter skin. Light skin colour is necessary outside the tropics to allow for vitamin D synthesis. Since the sun isn't as strong it is much less of a danger, and is ironically necessary. Too much vitamin D can lead to hypercalcemia. The darkest skin is found in the lowest latitudes of the open grasslands of Africa.

Evolution of the Homo genus

If you were to look at the skin of chimpanzees you would see that it is lightly coloured, yet the skin is covered in fur. It is the fur that protects these organisms from the sun. When humans branched off from chimpanzees they retained a few characteristics, light skin being one of them. As the genus Homo started to become more evolved, the body needed to make some adaptations. There was a migration from the forests into open land for the search for food and water. When more efficient hunting and gathering techniques were attained, there was a substantial increase in the metabolic activity of the body. An increase in the number of sweat glands was necessary as a means of cooling down the body. Having fur would be an inconvenience because the sweat would get stuck as the base of the hair follicles, resulting in an insufficient cooling technique. As a result, the body became remarkably more hairless, looking naked from a distance. With this adaptation came a drawback; there was virtually no protection from the UV rays of the sun. Being exposed to the sun for such long periods was of great danger.