Jolien,
I'm not sure which of my colleagues at the RAF Museum supplied you that information, but I can assure you that they were incorrect, and possibly misunderstood what was being asked.
Firstly the RAF had long stopped using blue-grey by the time the C15 was introduced into the RAF.
Secondly, even assuming it was blue-grey, the roundel on the front offside wasn't officially introduced until April 1941, around the time some commands were camouflaging their vehicles.
Thirdly, the command letter was introduced in August 1941 at the same time the RAF mandated all vehicles would be camouflaged. The group number that came after the command letter wasn't introduced until January 1942.
Also, there would not have been roundels on the sides of the body, and the only roundel on the rear would be a small one on the nearside the same as that carried on the front offside wing.
If your example has evidence of RAF blue-grey paint on it, then this is likely from it's post-war service.