the main reason is that a bruise consists of blood that is puddled beneath the skin. when you get a bruise, you break blood vessels and this is what causes the pain and discoloration. now, if you've ever cut yourself before, say with a paper cut, the blood is bright red yes? but when it scabs over it turns to a darker red and brown? well its nearly the same thing with a bruise. no, there is no scab inside of your body really, but the blood vessels which had broken have healed and the blood that was released is left where it is. now, you know that blood when exposed to air turns red, yes? and that when it is removed from air, it turns dark, or even blue? well, this is what causes the color change. as the blood works to organize itself back into your body, the lack of oxygen causes its color to change. basically, it goes from red, to purple, to bluish black and then brown... i hope this helped.