Historical Rules · 15 July 2011
Rule 903.4
Official source text from the 15 July 2011 Rules release
The color identity of a deck's commander 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). 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 converted mana cost to target creature or player." Bosh's color identity is red.