Colour identity crisis

[Kaworu, the Fifth Child]

My friend is running a {Tristani, Selesnyas Voice} EDH deck, and were having a debate over if {Garruk Relentless} is legal because of the flip side.

[Giggle the Draco Genius]

The color identity counts both sides of the card.

You can see in the left side of the bubble a little black circle which indicates a black identity as the flipped side does not have a mana cost.

903.4c 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.

[MuggyWuggy]

My friend is running a {Tristani, Selesnyas Voice} EDH deck, and were having a debate over if {Garruk Relentless} is legal because of the flip side.

It is not legal for a Trosti deck.

Elbrus the binding blade is considered black in the same respect