Balthasar, have you seen Rhye's mod/tutorial for custom civ colors? He made some nice multi-color effects by playing around with the palettes.
One would think that giving a (well, I guess any) unit 0 defense and making it surrender like a worker would essentially make it neutral so that it wouldn't "die", it would just "surrender" to every unit that comes by, ensuring that a stop light would remain "alive" throughout the game. Come to think of it, the "surrender" could turn the light green. . .