Do you like the idea of having several different hud colors to choose from? I think white, blue, yellow, or green would all be handy. Pretty much any light color would work.
Color wheel.
agreed. They could have 3 seperate color wheels. One for normal, enemy, and friendly
or just put a magnet up to the tv 
I was thinking of this the other day. And maybe you could change the shapes of your hud
also.