Since I shoot white center on dark green targets I guess I do not have to worry. Although with open sights at a longdistance, even that combination can also be a problem. My favorite is a black bull with a bright red or green center circle. It really sticks out well.

Something you can do for cheap targets.. go to Wal Mart and get some 3X5 florscent colored index cards. You tape them to a cardboard box and they stick out like a sore thumb. If you're shooting open sights the card makes a good aiming point. If your shooting a scope just take a black magic marker and make an X corner to corner on the card and you then have something for your scope to focus on.