Skin color explanation?
The genetics of pigmentation is relatively complicated, as the pathway for the pigmentation (regulation of the pigment production, ratio between the melanins, maturation, trafficking and distribution of the melanins from the melanocytes to other cells) is quite long and also subject to different regulations. All the mutations found to date (at least to my knowledge) affect single parts of this long process (so for example MC1R mutations are affecting whether eumelanin is made, while Rab7 and Rab27a is most likely involved in the distribution of the melanosomes). Not all gene functions in this network have been understood so far.

Population studies have shown that the three main ethnic subgroups (west-africans, asians and europeans) have developed independently from a common ancestor with different (and independent) mutations in each arm. See the figure from reference 1 as an illustration, it also shows the genes involved in each arm:



Pigmentation is a protection against UV light, so there is a strong selection against mutations in areas with high UV light (west africa for example). The general theory is that in areas with less UV light the skin got lighter to allow the production of vitamin D (for which you need UV light). If you want to go through the single genes and their influence on pigmentation, please have a look at this Wikipedia page, which gives a good explanation. Also have a look at the references.

References:


Unpacking Human Evolution to Find the Genetic Determinants of Human
Skin Pigmentation
The genetics of pigmentation: from fancy genes to complex traits.
Molecular genetics of human pigmentation diversity
Human pigmentation genetics: the difference is only skin deep
The etiology and molecular genetics of human pigmentation disorders