I literally only buy 1 color of socks -- a light gray. They're fine for casual or semi-formal, as well as any kind of exercise. I never have to look for a match, or try to decide what color pair I want to wear. And every few years I buy 3 or 4 (or more) packs of the same to replenish my stock.

Best of all, if I lose one sock, I never miss it; same thing if one sock gets a hole and has to be tossed. So socks technically cost me half of what they cost everyone else :)