Rule 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.