The light on Macbook Pros is just the other side of the backlight on the screen. If it was red or green, the screen would also be red or green.<p>This also means you set the brightness of the screen to zero, then get a torch, shine it through the apple on the back, and see what is on a small portion of the screen.
It's a design decision. Of course it could have been red or green but those colours simply don't look as good together with silver / metallic grey.