I have it on very good authority that the colour was chosen by the joint-services team at Yeovilton, responsible for the leasing contract for 'Huey Green' or 'Kermit' (or whatever!) to MoD from FB Heliservices Ltd.
I'm not sure, however, that their interpretation of the B&Q colour chart quite matched the reality......
I can recall my first sight of G-BGLJ/ZJ969 when I went to collect said aircraft from overhaul and respray up in Stockholm. It sure brightened up the day!