Yeah, you need some sort of contrast that is visible. You will find that at different times, on different days, the plane will appear brighter than on others. It all has to do with the contrast with the background. An overcast day will show different from a bright sunshine day.

There are so many different combinations, but, as said, the contrast must be drastic, like black and white.. yellow and blue, red and white, something that is really visible. But, as DaRock pointed out, this all becomes a mute point after a certain distance where they all seem to look alike.

One thing I try to do besides contrasting colors and large stripes (no, the checkerboard squares are not visible 500 feet away) is if you get slightly disoriented, wag the wings slowly, right to left. If the plane with the wing closest to you wags first down then up (right then left) then it's going away from you. If it's upside down, well be very careful!! [:@]