Whenever Aisling Leprechaun blocks or becomes blocked by a creature, that creature becomes green. (This effect lasts indefinitely.)
10/4/2004 Creatures change color as a triggered ability upon the block being declared.
10/4/2004 The change to green does not end even if the Leprechaun leaves the battlefield.
10/4/2004 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).