Most visiable color to cover plane
White is the brightest, most visible color for my eyes, especially on the leading edge of the wing. I used to have a problem when viewing a model when it was way out and coming right at you. That's about the smallest profile you can get (or, going directly away from you). It was especially bad around dusk. The plane would seem to blend in with the sky. White helped me.
I've got a 2 meter glider I fly a lot that is painted mostly bright yellow with black and orange trim. I use a power pod and flights usually make it up to almost out-of-sight (I've got to be careful or it WILL go o-o-s!) Thinking it would make it really stand out, I painted a checkerboard pattern (black and bright yellow) on the bottom of the wing. I was surprised to learn at what a low altitude the checkerboard pattern becomes completely unnoticeable! Just sorta looks like a dull yellow in no time as it's climbing.