That joke is a museum exhibit The other sides joke was..
“You can tell the nationality of an aircraft by the colour it is painted. If it is camouflaged it is British. If it painted silver it is American and if you cant see it all it is German.
There has not been a systematic historic study of friendly fire since 1982. Here is a m,oral question for you.
If attacked by "Friendly aircraft" do you shoot back? You might shoot them down but any return fire will spoil the pilot's aim.
dont need to, the RAF shoot up their own equipment