My favored color scheme (for sailplanes) is transparent yellow with a red-orange left wingtip and a green-blue right wingtip. Sailplanes can be specked out and this color scheme is very visible (the trans yellow looks almost fluorescent at altitude) and the red/right wingtip helps with orientation. It also helps to put the aluminized mylar holographic tape on the leading edge; the flash helps to spot the bird while circling in a thermal.

For my powered planes, I tend to like a yellow wing top with a red or blue wing bottom. Contrast is the key. I tend to favor yellow because it is "minus yellow" on the color wheel.