[Buckeye30]

Hi James Middle Stone (or Midstone) was certainly in use in 1940 but not "officially" until AMO A.926/40 of 12 Dec.1940--------
"I. 5. Para. "i" : Temporate Land Scheme camouflage.- This consists of two colours, DARK GREEN and DARK EARTH. Aircraft of the Middle East Command are coloured MIDSTONE in place of the DARK GREEN."
This would be a factory finish from then but reports from Malta in June and August 1940 ( when Hurricanes were being ferried there) indicate the Desert Scheme on some aircraft, the MS painted (usually) over the DG. They also mostly had tropical filters and black spinners .
As mentioned before, overseas commands could adapt schemes where they considered necessary.Some transports in N. Africa seem to have a sandy light earth colour with the Dark Earth.
"Sky" ( Duck Egg Blue ) was not official until June 6 1940 at the start of the BOB, and the older underside factory schemes ( black port wing, white starboard wing and aluminium lower fuselage--but with many variations) could be seen on some overseas day fighter after that including Cyprus and N. Africa ---and possibly later in Greece. There is an air-to-air photo of a Hurricane I said to be over Crete early 1941 in this scheme.
Note that "Sky" and "Duck Egg Blue" were one and the same to the Air Ministry, and "Duck Egg Green" was also used especially by the Observer Corps and Home Defence, this was a more apt description of the colour.

For the record these are the specified undersurface colours in the Ministry Standards (also included Aluminium and Night (black) ).
Please note not exact shades due to settings etc.and reproduction. Sky was lighter .
Nick