Emiritus, evansb, checking through my copy of "Fighting Colours" by M J F Bowyer, he states, at the beginning of Chapter 6, that during December 1940, several changes were made to the colour schemes of home-based day fighters. One of those changes was the introduction of an 18 inch wide band around the rear fuselage, just in front of the tail. The colour of the band was to be Sky. At the same time, it was also decreed that spinners were to be painted the same colour.
Hope that helps.