You don't see the contrast in the air, per se. What the contrast allows is that you see the light colors against a dark background and you see the dark colors against a light background. So regardless of sky conditions you'll at least see something. The contrast should also be obviously different topside vs. bottom so you can tell at a given moment which aspect your model is in in relation to your viewing angle. (i.e. stripes on the underside vs. solid on the top, or whatever). You'll notice most serious pattern ships have large checkerboard undersides and stripey, colorful topsides.