there are also some other factors. Your lighting setup when shooting against green screen is important. heavy shadows on the green wall behind the subject can mess it up. make sure the lighting is even, and try to eliminate the shadows on the green wall. also if the subject is too close to the green backdrop the subject will pick up the green color.

Also i cant remember why, but I remember that shooting on Beta gives a better result. something about how the color is done on a Beta Camera. Ive shot green on this type of camera with excellent results, and in the same studio shot on DV with not to great results.

here some tips:
http://www.vce.com/bluescreen.html


jamie