Rule 903.11 · Commander
Except via rules, special actions, and effects that specifically bring cards into Commander games from outside the game, traditional cards from outside the game cannot be brought into a Commander game.
Subrules
If a player is allowed to bring a card from outside the game into a Commander game, that player can’t bring a card into the game this way if it has the same name as a card that player had in their starting deck, if it has the same name as a card that the player owns in the current game, or if any color in its color identity isn’t in the color identity of the player’s commander.