Color identity is a concept expanding the color of cards. While in game play mechanics the color of a card is always determined exclusively by the colors used in its casting cost or its color indicator (which was retroactively applied to cards like Ancestral Vision, which previously specified it in the rule text), the color identity of a card is specified by all mana symbols that appear in its casting cost, color indicator, and rules text. This is primarily relevant for the Commander format, in which players are not allowed to use any cards whose color identity isn't encompassed by that of their General.
For example, while Fires of Undeath is a red card, and Silver Knight has protection from it and White Knight does not, it can't be used by a player who uses Kumano, Master Yamabushi as his or her General in a game of Commander. This is because the color identity of Fires of Undeath is red-black and is not a subset of the color-identity of Kumano (red).
Phyrexian mana is considered to be the mana of the color it replaces, e.g. Dismember is a black card. Hybrid mana is considered to be either color, thus Kitchen Finks has a color identity of white-green.
A special case is the hybrid white-black mana symbol on creatures with the Extort ability. It is considered part of the reminder text, not part of the rules text, so it doesn't determine color identity.
Rules
From the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)
- 903.4. The Commander variant uses color identity to determine what cards can be in a deck with a certain commander. The color identity of a card is the color or colors of any mana symbols in that card’s mana cost or rules text, plus any colors defined by its characteristic-defining abilities (see rule 604.3) or color indicator (see rule 204).
Example: Bosh, Iron Golem is a legendary artifact creature with mana cost {8} and the ability “{3}{R}, Sacrifice an artifact: Bosh, Iron Golem deals damage equal to the sacrificed artifact’s mana value to any target.” Bosh’s color identity is red.
- 903.4a Color identity is established before the game begins.
- 903.4b If a commander has a static ability that causes a player to choose its color before the game begins, that choice applies during deck construction and throughout the game, even as the commander changes zones. That choice affects the commander’s color identity. The player reveals that choice as they put their commander into the command zone before the game begins. See rules 103.2c and 607.2p.
- 903.4c Reminder text is ignored when determining a card’s color identity. See rule 207.2.
- 903.4d The back face of a double-faced card (see rule 712) is included when determining a card’s color identity. This is an exception to rule 712.8a.
Example: Civilized Scholar is the front face of a double-faced card with mana cost {2}{U}. Homicidal Brute is the back face of that double-faced card and has a red color indicator. The card’s color identity is blue and red.
- 903.4e If a card has any alternative characteristics, such as those of adventurer cards (see rule 715, “Adventurer Cards”), those characteristics are included when determining the card’s color identity.
- 903.4f If an ability refers to the colors or number of colors in a commander’s color identity, that quality is undefined if that player doesn’t have a commander. That part of the ability won’t do anything. Costs that refer to that quality are unpayable.
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