Creature — Faerie
Whenever Aisling Leprechaun 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).