Chromatic Anomaly

Whenever an opponent taps a land for mana of
a color that hasn't been added this turn, put a
+1/+1 counter on Chromatic Anomaly.
At the beginning of each end step, if Chromatic
Anomaly had three or more +1/+1 counters put
on it this turn, exile all cards from each player's
hand, then each player draws that many cards.
"It was just a colour out of space"—H.P.Lovecraft