EDH/Commander Deck Construction Question: Transformation Card Color

So, I have a pretty darn good EDH deck that I've been using and improving for a while. My commander is Captain Sisay, which means that I can only have colorless, green, and white cards in my deck. However, since her ability can search for legendary cards, I would like to put Elbrus, the Binding Blade in my deck. Would this be legal? Although Withengar does have a black color indicator, Elbrus colorless. I tried searching for the answer but there doesn't seem to be an official ruling that I can find. Can you help me, r/magictcg?