For me, it depends on the car- honestly. For more sporty cars, red is a great color. We used to have a Red Porsche Boxter, and now have a red Mercedes CLK320 convertible.
That being said, my car is technically called "Champagne"- which is a light tan-ish, cream color, and for that car (a Saab 9-2x) it looks perfect. Looks better than that car in a red.
Black always looks sexy on any car though, too!