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Hmm, I bought Humbrol 90 for my Hurricane I, it is called for by the Airfix instruction sheetIt does look too green to me, I must admit, but it's a long way to my local model shop and the undersides are getting done tomorrow night after work
Humbrol 90 = Tamiya XF21
That's the one, No.23 ! There's always been a little confusion over 'Sky', as there was 'Sky type S', and later,as Wojtek said, 'Sky'. Both were called Duck Egg Blue or Duck Egg Green! Generally, I use No.23, although I haven't any at the moment, so just mix my own. If you're bothered about the shade BT, just add a small touch of white, and it'll pass! I wouldn't worry anyway, as there was so much variation up until late 1940, that the shade could look cream to greenish to blueish to yellowish!!
That's why the 'standard' became 'Sky' which, by then, was really only used for code letters and the fuselage bands, as well as some 'not as common' camouflage schemes.