According to Wizards of the Coast, Magic:The Gathering deck types are classified by the color of the cards used in deck construction. The colors available include white, blue, black, red, green and colorless. Colorless cards are usually artifacts or artifact creatures that have unique abilities, mimic abilities of another color or no special abilities at all. Their mana cost can be paid with any of the five types of mana.
White cards focus on protection, healing and annulment. These cards provide damage prevention for the player and his creatures, recover health and negate opponents' advantages. However, this color has difficulty destroying creatures and dealing damage, and many of its spells affect all players, including the player who cast it.
Blue cards are about hindering and stalling opponents while a player builds his attack. This color is best at countering opponents' spells and making them discard cards, while giving the player more options through drawing more cards or using the special abilities of its creatures or enchantments in play. Drawbacks include its generally weak creature attack and the high mana cost of its spells.
Black cards focus on limiting opponents' attacks while going in for a swift kill. Cards of this color destroy opponents' creatures, deal direct damage to the player's enemies, and feature creatures that are often unblockable. While black cards are powerful, they often require sacrifices of the player's own life or of his creatures; back cards have trouble dealing with artifacts or other black cards.
Red cards follow an aggressive style, featuring a fast attack of creatures and spells. This color of card has creatures with high attack power; and it often attacks as soon as it is played. It directs damage spells and enjoys a typically low mana cost. However, its creatures are easily destroyed, their high attack is often temporary, and Red cards do not have many good defensive options as the game progresses.
Green cards focus on building a powerful creature attack that overwhelms an opponent. Green creatures are generally strong and difficult to destroy, and this color spells can increase the caster’s mana pool, add power to creatures, and regenerate life. But, the green cards wide-open style can leave its player open to attacks from spells and creatures that have unblockable abilities, and these cards do not slow down opponents.
Decks can be built from just one color, or from two or more colors. It is easiest to mix in colorless cards in a deck, as no additional mana types are needed. However, it is often beneficial to work in more than one color to balance a deck. Adjacent colors on the Magic: The Gathering “color wheel”: WUBRG have complementary styles. Blue is abbreviated U, and Black is B. The two colors on the ends, White and Green, are also considered complementary. Experimenting with different combinations can yield very powerful decks.