DZ, I just don't see why it would have been painted like that. As I said before, any aircraft destined for fire/rescue practice rarely gets repainted in flight-standard finish like that JP appears to be. They generally get a simple (and pretty rough) application of whatever paint happens to be available. A properly-applied camouflage with LAG undersides and full insignia isn't something that anyone would bother applying to a fire/rescue hulk. The application of proper serials also doesn't add-up. The Maintenance number should have been applied.
So, I'm inclined to think that the aircraft must have flown in those colours at some stage but I can't imagine why. It was long before Brawdy came along and started flying JP's, indeed at that time it was only Chivenor that would have had any aircraft in the FAC role, and they were still using Meteors and a Chipmunk (in grey/dayglow) at that time. If the aircraft's previous base had been Cranwell, then it would have been painted in the standard silver/dayglow scheme or possibly the red/white/grey scheme that was just starting to be introduced. But camouflage? Having examined the photo though, it does look as if there's some trace of orange under the base of the rudder and on the nose? If so, that does suggest that the paint wasn't applied all that well, so it could have been intended as only a temporary finish?
It's a complete mystery and potentially a very interesting one as I've never seen any mention (or picture) of this before. Come on, someone out there must know what it's all about?! If it was painted-up like that just for fire/rescue work, then they must have had some very enthusiastic (or bored!) paint finishers at Thorney Island!