The way I see it is this:
Blue is Trickery.
You keep your opponents spells out of play, either by countering, or removing, nullifying... etc. You also generally mess things up... mess up libraries, hands, perhaps graveyards, and playing fields. You poke with your Prodigal Sorcerer which can kill an opponent in 20 short turns.
Black is Necromancy
You kill stuff, you bring stuff back, you raise undead minions. You apply 'curses' which cut power and toughness, you do some pretty evil stuff. Some of it is so evil you make sacrifices to deal heavy blows.
White is Defensive
You protect (CoP ftw), you heal, you revive. You enchant and disenchant the playing field. Enchant it with effects that give you life, disenchant those evil Black and Blue ones that mess everything up. You provide walls. Most importantly, you heal and prevent damage.
Green is Monstruous.
You have some really effing big creatures. Those cards you have that aren't creatures, heck, you can turn them into creatures too. Those not so effing big creatures... well, let's enchant them and make them bigger. And better. Rawr.
Red is Damaging
Sure the creatures aren't as big, but there's more than one way to drain life. Play sorceries to directly hurt your opponent and their creatures... destroy lands. Destroy stuff generally. Craft weapons that give your creatures more power.
Of course, these aren't set guidelines. Blue can provide some good walls and some good life recovery. Black can have some big Horrors, Green tends to do a bit of recovery and graveyard-creature work. They all tend to dabble with tokens.
Multicoloured ftw~
