Historical Rules · 1 July 2012
Rule 903.4c
Official source text from the 1 July 2012 Rules release
The back face of a double-faced card (see rule 711) is included when determining a card's color identity. 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.