As far as I can tell from the limited references I have, Sturmoviks were always finished with blue undersides. Late in the war, some fighters had a light gray undersurface, but not IL-2s. Sturmoviks seem to have been usually in the black and green or two-tone green uppers, with whitewash applied in winter, either overall or in bands. Accurate Miniatures has some info on their instructions for IL-2 kits. As an aside, IL-2a were worked very hard in primitive conditions, so would often be very weatherbeaten. Weathering for wood wings would be quite different from metal, though, and reference material is hard to find. I suggest you look for a Russian source.