Stone of Purity

Whenever a player uses colored mana to pay for a
cost that could have been paid with colorless
mana, they take 1 damage. (For example, if a player
pays a {4} cost with {c}{w}{u}{u}, they take three
damage.) At the beginning of each player's end
step, they take damage equal to the number of
colors of mana they spent that turn.
In the beginning, there was purity, and in the end,
there is conformity. Only in between is the chaos that
will be beaten into submission.