Yokgu, the Unifier

When Yokgu, the Unifier enters the battlefield,
look at the top seven cards of your library. Reveal
up to 2 of them, and put then in your hand. Then shuffle your library.
Whenever you cast a creature spell, add {x} mana
of your choice, where {x} is equal to CMC
of casted creature card.
You can't cast single colored creature spells.