Zekrom, Born of Thunder

Flying
Whenever you cast a spell, put a +1/+1 counter
on Zekrom, Born of Thunder for each of that
spells colors.
Remove five +1/+1 counters from Zekrom: Add {w}{w}{u}{u}{b}{b}{r}{r}{g}{g} to your mana pool.
Activate this ability only once each turn.