Yes. Color identity is only determined by mana symbols in casting costs and activated abilties. You can use Tatsumasa, the Dragon's Fang in a mono-white deck, same way as you can use Nim Deathmantle in a mono-white deck, or Scalding Tarn in a deck that only uses U or R mana. Words don't count towards color identity; mana symbols do.
June 30, 2015 1:08 p.m.
It should be noted that color identity also includes color indicators and characteristic-defining abilities. It's not limited to only mana symbols.
June 30, 2015 2:07 p.m.
Also, reminder text doesn't count towards color ID. You can run extort cards in mono-white or mono-black for this reason.
June 30, 2015 2:13 p.m.
Any mana symbols not in reminder text count; I realize the first answer says "activated abilities," which is not accurate.