I wear amber lens sunglasses over my regular glasses when flying. They enhance contrast and really make colors pop against the blue skies typical for where I live while simultaneously protecting my eyes from the blinding desert glare. The Ultracote Bright Yellow on the bottom of my Tiger 60 practically glows, as do the white numbers on the tops of its wings. Good thing, too, because the Deep Blue upper covering is a bit less visible against said sky - a minor brain fart on my part when picking colors for it.

Other colors that I've seen work well are neons in green, orange, yellow and pink. These are popular colors for model rocket parachutes around here and are easily seen against the sky.