Previous Message

Not one but both. Judging from the underwing float and the underneath of the top wing, it appears to be Sky overall. An alternative could be white for Arctic service but both would be unique.
I suspect that it has been "bulled up" for the King's visit so any available paint or distemper will have been used. The only light grey paint was the Sky Grey not used for two years, but it seems too light for that.

I posted without seeing Ian's comment on the demarcation. I can see that too, and agree that on some film/filters there is very little difference between the two top colours, and both appear light. This case does appear especially light. The Temperate Sea Scheme can be very misleading in b&w photos. (That's my excuse.)