Are you trying to decide whether to use a blue or green or red or whatever screen? Go with a color that is the opposite of the foreground color. Blue or green backgrounds are often used because there is very little of those colors in human skin. If you were shooting a product that has a lot of blue and green in it though, you might be better off using a red screen.
Make sure that the screen is evenly lit, and that the foreground person or object is far enough away from it (with its own lighting) so that there is minimal color and shadow spilling (and reflecting) between foreground and background.
When lighting the foreground (the object or person that will not be keyed out), try to match the lighting of the replacement background in the final composite. For example, if you're shooting an actor against a blue screen, and you intend to delete the blue and place the actor over a Mars landscape, make sure that the light is shining from the same direction (and with the same color and intensity) as the light shining on the Mars background.