It's a Royal Air Force Tactical Recognition Flash. The soldier in the picture is probably RAF Regiment.
The TRF is used to identify the wearers Regiment or Corps and, as has been said, the Regiment or Corps Stable Belt colours are often used.
They should not be confused with formation Signs, e.g. 7th Armoured Brigade - a red jerboa (or Desert Rat) on a black ground. Or an old one that is one of my favourites 56 (London) Division - always described as : "a black cat with sword up its bum".