Was it always green? What were you doing before it turned that color?
i'm guessing that since you're on the log in screen, you've restarted a few times already?
when my mom's laptop did that, i sometimes could slap it back into its right colors.though i don't really recommend that if you're at school.
How old is it?It may be a problem with the screen's "color guns."When the color guns that create the monitor's display begin to fail, the result can be an alteration in the amount of the color over part or all of the screen. The green color gun is especially prone to failure, as the green color gun carries the main video signal for the monitor.