In the color-coordinated world of an aircraft carrier flight deck, those wearing the purple shirts are those guys and gals that take care of the fuel-related aspects of the carrier. Called the Grapes, these folks make sure the aircraft get the fuel they need or in some cases get de-fueled so an aircraft can go down below into the hangar bay.
Every once in a while a jet might end up at an USAF base before coming back to the ship and may have a belly full of the fuel the AF uses - JP-4, vice the fuel the Navy uses, JP-5. JP-4 has a lower flash-point (the temperature where it can vaporize to form an ignitable mixture in air) than JP-5, which is why the Navy does not use JP-4 on the ship (You want to avoid the possibility of your ship catching fire at all cost. The alternative? How long can you tread water?) Any aircraft that lands with JP-4 on board needs to be de-fueled to get that evil USAF gas out. The Air Department's fuels folks, the V-4 Division, takes care of this. Here's a good article from Navy.mil on what the USS Abraham Lincoln's V-4 Division does.
Here, two Grapes are discussing some aspect of fueling one of the USS Harry S Truman Hornets. I was up on the starboard side of the ship, a tad forward, looking back toward the island. The aircraft are parked "TOW", or "tail over water" to help maximize the parking/positioning of aircraft on the relatively small four and a half acres of flight deck.
On board USS Harry S Truman, summer, JTFEX, 2009
Update: I am *really* dating myself here. Had an correction from a reader:
The Air Force hasn’t used JP4 since 1996. It was replaced by JP8 which is very similar to JP5. The lower flash point of JP4 was one of the reasons for the change fifteen years ago.
I left active duty in 94 and I should have been asking more questions about the state of play with fuel when I was on Truman for my last reserve duty. Hat tip for the correction to MSgt Evans! Appreciate it! File my original JP-4 comment in the "Used To Be" category.