The following, I feel, are the most intuitive:
I agree with those who suggested a silver or white color for Abjuration. Abjuration is protective magic. In most common magical theories and thought, white or silver has always been assigned to protection or 'good'. Even in some other pursuits, such as some so-called psychic healers or other such mystics that give advice on protection (psychic, mystical or otherwise just for peace of mind/meditation), they often suggest envisioning a white or silver light. The silver cord of astral travel is a kind of protection that allows you to get back to your body. In the Magic card game, the most protective magic color in the deck is often white.
For similar reasons I would also think of Necromancy as Black.
Evocation... red - for the flames and energies that are commonly manipulated. Face it, when you talk evocation Fireball is one of the most popular and widely known spells. If you wanted to continue to use M:tG as part of your argument most direct damge 'spells' or cards in that game were red for a time.
Illusion for some reason strikes me as a shifty, shadowy, murky pursuit. It's used to hide or deceive. This lends me to think of it as a darker color, such as purple or a very deep blue.
Divination, perhaps a light blue. Discovery and knowledge is generally considered a positive thing, and blue a positive color. There are other things that make this color intuitive for divination, such as the phrases "it came out of blue" or "then it hit me, from out of the blue" or "inspiration hit him like a bolt of blue". Further, one of the main inspirations for divination is dowsing, which was used to locate water, also commonly associated with the color blue. Again, the designers of M:tG seemed to think the same way because Blue had all the mental spells like Telepathy.
Enchantment, I feel, would have a green hue to it. These nature colors would go well with a school that has spells that enhance what some would call 'natural' charisma or charm-like effects. Enchantments, positive or negative, were often the cause of sprites, nymphs, dryads, and pixies... all associated with a sylvan setting, again lending itself to green. Again, the designers of M:tG seemed to think the same way because a lot of the enchantments in M:tG were in the green cards.
I'll think about Transmutation and Conjuration for a while, but my first instinct is gold for Transmutation - one of the major goals of alchemy (the psuedoscience or fantasy science of transmuting) was turning lead to gold. Also, transmutation deals with changing things, often to give them added value or potential. Again, this leads me to think of gold.
