Certainly the term 'Sky" was in use early on in the peace (pre war), and early WWII.
However not all, is so simple, as there were other colours in the melting
pot so to speak, such as 'Eau De Nil" and terms as Camotint, which existed
also and had more of a direction to the eventual "Sky Type S" as we know it.
I would suggest reading this article, the gentlemen who writes this blog is very
well researched and in my opinion quite authoritive.
amair4raf.blogspot.co.nz/.../blue-skies-and-duck-eggs.html
With respect to the references to the RNZAF, not so cut and dry.
The RNZAF had two versions of Duck Egg Blue
1) (NZ) Duck Egg Blue: BALM S13-909. 33B/N.90
2) (NZ) Duck Egg Blue (this one darker than the previous)
In his book 'We Also Served", WWII veteran Wally Ingham (RNZAF Air frame fitter) made mention that parked out on the strip, there were many aircraft all Quote" painted in Duck Egg Blue,
like the RAF" end quote.
As to the P 40's and Venturas and Hudsons wearing Duck Egg Blue, many of the
early P 40's (E version and some M versions) Hudsons and Ventura's arrived from the United States in US "RAF" versions of Camouflage being Dark Earth/Dark Green/Sky.
To make understanding easier, I will deal with the P 40 E's, but comments for Hudsons etc will be similar.
In early 1942 the RNZAF received a shipment of P 40E's from British Stock held in the USA.
The Aircraft were painted in RAF colours using DuPont paints that were a version of RAF colours, being Top: Dark Earth/ Dark Green and Sky lower (per Curtiss documents held by RNZAF Museum).
The DuPont colour for Sky (US version) was DuPont 71-021 Sky Type S Grey.
This paint was a Bluish/Greenish colour with a grey tint and was colloquilally called Duck Egg Blue in the US and would have for the RNZAF as well.
This restored P40E shows how the aircraft would have looked circa 1942, note the lower colour
i40.photobucket.com/.../FILE0263copy_zps5a2334d5.jpg
The Duck Egg Blue (manufactured either in New Zealand or Australia under the BALM series)
would have been close to the US version for repairs etc.
The RNZAF did use Sky for Spinners and aircraft codes, some of it's aircraft for example No. 14
Fighter Squadron P 40E's, had "Sky" spinners and Codes. Ths 'Sky" would have been very similar
to RAF Sky AKA Duck Egg Blue.
Later in 1942-43 RNZAF Aircraft began to be camouflaged in New Zealand colours of NZ Blue Sea Grey/Foliage or Dark Green/Sky Grey. This generally only applied to New Zealand based aircraft.
Later toward the wars end, in New Zealand, aircraft began to be painted in Foliage Green/Sky Grey.
(this also applied to returning P 40's from the Operational areas used for training)
(NOTE: Other lend lease aircraft, wore what ever colours they arrived in, in the forward operational areas (TBF-1's P 40(M-N) SBD's F4U's etc)
The Sky Grey, interestingly had a similar hue to the Original Sky Type S - Grey, that the previous US built aircraft wore on arrival.
Videbeests/Vincents/Oxfords ect, would have worn Duck Egg Blue.
The only other green (non camouflage) found on WWII era RNZAF aircraft was a matt green for ID purposes.
Harvards wore either Trainer Yellow or Dark Earth/Dark Green/Yellow.
Regards
Alan