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Whenever this creature blocks or becomes blocked by a creature, that creature becomes green. (This effect lasts indefinitely.)
Creatures change color as a triggered ability upon the block being declared.
A Leprechaun with Protection from Green can be blocked by a creature (since it is not green at that time) but will not take damage from the creature (since it is green during the combat damage step).
The change to green does not end even if the Leprechaun leaves the battlefield.