on a sunny day even white is dark on the bottom of wings and horizontal stabs....you can see the colors best when the sun hits them as in a bank or high angles of attack....therefore I like to put on a stripe or make my rudder different than the fuselage..and a nice stripe on the bottoms of the wings..plus I put my AMA number on the top of the left wing like the old registration number.
as for colors..orange, yellow, or hot pink show real well