These are questions that have perplexed even the greatest of theologians. The Bible itself does not give us an answer as to where Cain and Seth got their wives. Non-Biblical Jewish sources indicate that they were their sisters.
The first indication that we have of different skin colors is in the sons of Noah. I say indications because this is not clear also. It is believed that one was white, one was red and one was black because their names meant just that.
Looking at new scientific evidence the color of a persons skin is determined by cells called melanocytes. All races have the same number of these cells. Melanosomes are structures in the melanocytes that produce the pigment melanin. There are more and larger melanosomes in darker-skin melanocytes than in those of lighter skin.
A good link to look at this more indepth can be found at:
http://www.wordoftruthradio.com/questions/skin.html
