[Graham Boak]

Azure Blue was allocated an official RAF stores number in December 1940. This followed the use of a similar colour created in the Middle East for local use, as supplied to the UK authorities on 21 November 1940 with a request for its official adoption.

So the actual use of Azure Blue (or a close match) considerably predated the issue of the Air Ministry Order officially approving its use, which (IIRC) was in August 1941. Apparently Sky Blue was also used around this time, although the wartime colour appears to be too pale and the prewar colour of the same name looks much more likely (if rather odd!). The MUs in the Nile Delta also used the darker colour Mediterranean Light Blue for undersides. The AMO in 1941 should be seen as regularising these practices rather than introducing them, as more normal in the UK.

I refer you to Issue 3 of Scale Aircraft Modelling, where Ian Huntley discusses early Middle East colours, and the recent On Target Special No.2, Britain Alone, where Paul Lucas has three chapters on the subject.