For more information on colors in Magic: The Gathering, see MTG Salvation's article on the color pie.
The colors of Magic(white, green, red, black and blue) are the fundamental building blocks of the spells, the mind, and the multiverse.
The colors of Magic are subtypes and they also replace alignment and damage types. Characters start off with one color chosen from their race or class color list. After the first level, a character can only take levels in a class if they have a color subtype at least one of the colors from that class's color list. A character can't casts spells. initiate maneuvers or use supernatural or spell-like abilities that have a color subtype that the character doesn't have. Most characters only have one color subtype, but they may gain more by taking the Multicolored feat. A Character who acts with blatant disregard towards a color's philosophy, especially if they act in accordance with one of their color's enemy colors' philosophies, may have their color changed or removed at the DM's discretion, although only constructs and certain outsiders can be without any color subtype. If a color subtype is taken away, only preforming a color-appropriate act of penance, followed by the atonement spell can restore it.
White is the color of order, law, and teamwork. White magic has a focus on healing and protection. White is the best color you can be, because when everyone works together, everyone can prosper.
Green is the color of nature, instincts, and growth. Green magic can bolster others, create powerful beasts, and destroy the unnatural. Green is the best color you can be, because when you follow you instincts, you live a life free from worry.
Red is the color of passion, impulsiveness, and unmitigated freedom. Red magic can create fire and lightning, unleash frenzy, and shape stone. Red is the best color you can be, because only when you are free from the control of others can you truly be alive.
Black is the color of individualism, ambition, and usurping power. Black magic can raise the dead, drain life force from others, and invoke fear and terror in the hearts of its enemies. Black is the best color you can be, because it combines the desire to completely safeguard oneself with the power to make that happen.
Blue is the color of knowledge, intellect, and rational inquiry. Blue magic can conjure wind and storms, manipulate the minds of others, and interfere with magic itself. Blue is the best color you can be, because when you know everything, you can't make any mistakes.