Assuming we are talking about the pacific theater:
For a practical if not perfect solution for a 2-colour upper scheme in delivery scheme: US medium green and the tan from the USAF SEAC scheme, over a pale grey like USN light gull gray.
Some Es had a wavy demarcation line between upper and lowers; these aircraft had, I believe, the undersides repainted in RAAF sky to cover up "US ARMY" markings. I have been told that IDF pale blue is not too distant from RAAF sky, though perhaps a tad greenish. So I have been told.
Aircraft in single-colour uppers are OD over NG.
It gets complicated in the Med because no Es were delivered in desert camo, but had the green areas, but only the green areas, overpainted (frequently leaving a rectangle of the original scheme around the serial) - but I can't remember if with middle stone or a mixed colour.
I would suggest Ian K Baker's colouring book on the P-40.