Is there a reason for why were the five colors of MTG ordered, clockwise, white, blue, black, red and green?
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.