The eye loses color recognition way before it loses grey scale recognition. That's why a plane looks shades of grey when its some distance away.

Red and blue, for example, look exactly the same if they are the same 'darkness' from a distance.

So, shade difference between the top and bottom of the plane is more important than color difference.

I always try to give the model longitudinal contrasting shades on the bottom of the wing (large stripes, maybe 3 across the chord of one wing half running parallel to the fuselage) and span-wise large stripes on the top of the wing. As long as they are contrasting shades (black + white; dark blue + yellow, something like that) you can choose your own colors.

Also, it's a good idea to have the rudder that's a contrasting color to the fuselage, and maybe a contrasting shade (again!) split by a horizontal boundary halfway up the fuselage side.

-David C.