Green feet means rescue! The origin of the green feet symbol came from Vietnam, when the HH-3E helicopter, also known as the Jolly Green Giant, would land in the rice patties and grass fields, leaving huge impressions that looked like giant green footprints.
The green feet tradition arose during the Vietnam War, at which time the most commonly used USAF helicopter was the CH-3E, nicknamed the Jolly Green Giant due to its enormous size and olive drab exterior. The tradition came about when pilots, navigators, enlisted aircrew, or other military personnel were in need of rescue. After these personnel were rescued, they would proceed to receive the temporary ink-stamped "tattoo" of the green feet on their buttocks due to the fact that the Para Jumpers "saved their ass."
One of the most storied Pararescue traditions is getting the green feet tattoo, which traditionally is located on an Airman's posterior.