The origin of the color wheel
The colors adjacent to each other on the pentagon are "allied" and often have similar, complementary abilities. For example, Blue has a relatively large number of flying creatures, as do White and Black, which are next to it. The two non-adjacent colors to a particular color are "enemy" colors, and are thematically opposed. For instance, Red tends to be very aggressive, while White and Blue are often more defensive in nature.

I don't think that white is on the top has a reason though.