I wouldn't leave the top of the wing all white. If you think about it, the most common colors in the sky are white and blue. Another thing to consider is you only see the bottom of the plane about 1/2 the time. When the plane is farhter out you do a lot of turning back toward you. When you do that its the top part of the wing thats pointing toward you. If its just white and a long way out, then thats when you could lose sight of it! If you want to keep it simple and stick with white then you could do a bright outline of the wing like John suggests. Neon colors show up the easiest. I use a lot of Neon Yellow on my planes and some of my friends like to use Neon Orange. If you go with the outline idea try to make the edges aound 6" wide to make the ouline visible. The smaller the edges, the more muddy the colors become at a distance. The pic below is my 60 sized Sea Fury in Neon Yellow. Have fun, Fast!