It's not so much the issue of the relative brightness. You can key out dark areas just as well as bright areas.
What I meant to say is: if you don't light the subject with enough light, the green cast caused by the greenscreen is going to spill all over the white shirt - causing it to come out green in the image.
Hope that makes more sense now.
Bar3nd
Raamw3rk - digital storytelling and visual effects