What was said above was great advice, contrasting colors. That's the key to being able to distinguish your plane in the air. But.......... Even with contrasting colors there are some colors that need to be mentioned because they have problems.

Black - Plain and simple, black can be hard to see in overcast conditions or if you are flying near dawn or dusk. They black has a tendency to "disappear"
White - White planes are hard to see when it's cloudy
Silver- silver colors disappear in a heartbeat in all colors
Blue - blue sky, blue plane, you do the math!!
Chrome- I've seen a plane covered in chrome monokote that looked great on the ground, but was really hard to see in the air. Imagine flying a plane that looks like whatever the background it's flying against. A true stealth plane if you ask me!!!

I have found that the colors I can see the best are Red and Yellow. If you work those colors into your color scheme it should be easier to see your plane in the air. I recently did a review on [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=745]Model Tech's Mach Racer[/link] (which by the way is an absolute BLAST to fly) and the yellow on the plane saves it every day at the flying field. This plane is the only plane I've ever seen that truly has "unlimited vertical", meaning that it will climb out of sight (and still be climbing). Anyway, I pull this plane into a climb and when I lose it I cut the throttle back, count to 5, and then give it full right aileron. I then watch for the flash of yellow on the top of the plane to find where it is!!!!!!!

Ken