Hi Mal:
The information supplied to me with the test shot states "McNair's aircraft was reported to have been delivered to Malta in Dark Earth / Mid Stone and Sky camouflage but to have later had the Mid Stone over-painted with a locally mixed 'dark grey' colour".
Now, I wondered whether this reference to 'Sky' meant Sky Blue or Sky Type S. The camo diagram was clearly 'S'. In tandem with this, I've had the Special Hobby kit SH48051 Spitfire Mk.Vc "Malta Defenders" for some time. The fourth option in the box is a Volkes fitted Vc / BR344 / 3-M, reportedly delivered to Malta on 9 May 1942, in Temperate Land Camouflage of Dark Earth and Dark Green uppers and Sky lowers. The 'Sky' is presented on the painting diagram as the 'beige green' depicted by Airfix.
Conventional wisdom dictates that a tropicalised Vb would be wearing Azure Blue (if delivered in DE and MS), in line with what your post was driving at. Entirely fair comment. However, both Airfix and Special Hobby seem to have at least some clue to an anomaly here and if there's one thing the Malta situation throws up - it's that there are large gaps in our knowledge of what was applied to some of these aircraft and that that will likely be the indefinite state of affairs going forwards.
Regrettably, I'm not privy to the intel Airfix base their MS contention on. To me, it seems (given the Special Hobby findings) more likely that McNair's Vb turned up in DG / DE and Sky but as I say, I'm yet to see the case for the defence m'lud.
In the final analysis, modellers are obviously free to go any alternate route they wish, while my role is to present it as Airfix intended and let things unfold from there. The McNair scheme 'as is' was my brief and its contrast with the usual expected tropicalised finish was certainly a draw to that anarchic side of my modelling persona I guess - tee hee.
I'm glad you brought it up. ;-)
Best regards
Steve