I agree with Wurger. In the case of U.S. built aircraft, they were delivered in whatever colors the manufacturer was delivering to the various services. In service I don't believe there was any set national standard. I have seen pics of Curtiss Hawk IIs in 4 different schemes: OD fuselage and Yellow wings, overall OD, overall silver dope and OD upper surfaces with neutral gray undersurfaces. Of course the most famous during the war were the Curtiss 80A/P-40s of the Flying Tigers. These were taken from a British order so they were in standard RAF colors. So it really depends on what aircaft you are trying to model. Post the type you're planning on doing and We'll try to come up with more info for you. Hope this helps.