Hello Larry,
Most sources seem to agree that RAF blue was used pre-war. During the war the RAF painted its vehicles in the same colours as the army, depending on the theatre of operations. These two links might give you a bit more information:
http://www.mafva.org.uk/PDF/BritishCamouflage1.pdf
http://www.network54.com/Forum/47208/thread/1139819680/
Hope this helps
Fyll