During prep right before D-Day the ground crew painted around the insignia and numbers, so you'd want to put the stripes on first and then decals. Didn't think of mentioning it, but one way to show that the stripes are out of place is to weather the plane before putting them on and leave them totally clean minus panel shading. If you're going with Polyscale, I'd think "engine black" would do nicely - that paint is as black as Stalin's soul.
Eric