The
tempG = g;
in your paint method doesn't make any sense. The graphics object is created by AWT when it calls the paint method, and isn't valid anymore outside of the paint method. Later calling paint with tempG in ChangeScreen therefore doesn't do anything useful.
You should never call paint directly. Instead, if you want the component to update its appearance, you should call repaint() instead. That will cause AWT to call your paint method with a valid graphics object.
Moving to the AWT forum, where the experts will correct me...
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus