ALL MT in the RAF is established against a "role".
Those established for UK airfields whose Sqns do not have a tac role should be yellow. All recent contracts for such vehicles calls for yellow. older vehicles in green ( normally satin BSC 241) will be/ are being repainted as repainting becomes due. Those on units with a Tac role which could be deployed with that unit are mostly green.
There are also vehicles with a deployable role; These were delivered in Matt IRR Green to BSC 285 and some in Light Stone BSC 361 and some repainted into the new desert cam BSC 380 . Some of the Matt green ones have disruptive pattern black. Once upon a time the RAF had a reserve fleet of critical vehicles referred to as second echelon , they were in addition to the Geographic ( formerly command) reserves. The current policy is that the deployable fleet known as the OOA ( out of area) fleet and the geo reserves are held on units and kept in regular use. It has been found they are far more serviceable with this policy. The geo reserve is established to provide fill-in for vehicles under going repair or overhaul.
The Haz label on the bowser identifies its content; the bowser colour has nothingh to do with its content.