Here's a variation on the ROTJ poster with a green lightsaber. It's a bit tricky to make it look right. If the green is too saturated, it looks out of keeping with the rest of the poster, but if it's not saturated enough, it looks different from the way it looks in the film. Also, in green, it sticks out like a sore thumb in the composition. In a color theory way, it overwhelms the rest of the poster ... But I was curious to see what it'd look like. I hope it helps.
http://img.photobucket.com/albums/v510/wannabegeorge/rotjgreen.jpg