From what I recall, the late thirties and early War scheme would be a splinter camouflage of a mid-to-dark green (Dark Angels Green, for example) and a darker Green-Grey (Chaos Black, some Codex Grey and maybe a dab of Dark Angels Green) for the top surfaces, with a light blue (Lichtblau) for the bottom.
While you can easily use GW colours for the top surfaces, Lichtblau is a trifle difficult to mix (Bone White, Hawk Turquoise and a tiny spot of Jade Green might work), and I would strongly suggest shelling out the 2 quid (or so, depends on where you buy) for the Revell Aqua Color Lichtblau (it's specifically made to match the tone and comes very, very close, from what I hear). You can use Revell Acrylics on normal minis as well (I do it all the time), so it won't be wasted. I would suggest a GW paint, but I don't think they make one anymore; the old Space Wolves Grey (in the old, round, soft pots) was very close, though, and if you still got that one, use it.
Alternatively, there's also a nifty three-tone splinter of light grey, dark green and dark brown used in the 1930s (comes up first page if you googleimage "HS-123). It also has a swastika band around the tail for that extra spot of colour (usual caveats as per swastikas on the forum apply).