There are a lot of really nice looking blues out there. Some lighter, some darker. Look at the 2009 Porsche Aqua Marine Metallic. Or the traditional Subaru WRX blue. There is the GM Imperial blue (kind of dark but brightens up in the sun as does the Porsche blue). Another blue is on the 2009 Honda Accord Coupe. It is specific to the coupe only. It is really nice too. That was suggested by Ben Hermance to me and I really like it.
Silver is always a good one, hides dirt and scratches pretty good too. And it looks great for years. It will always sell good too.
Charcoal greys. Some really dark greens. Really deep true reds always look great.
I went to about 15 car dealers one day when it was sunny out. I looked at everything I could and still was confused.
It's a tough call. Good luck.