What makes you think the stress colors are related to lack of poo? An adult tends not to poo so often. Have you seen the lizard straining/trying to poo, hanging its butt over a branch? What did the last poo you saw look like? Is the lizard drinking?
You could try giving it a knee high soak in tepid water or a shower, but unless its straining, three days should not be a concern.
All of the above. That said, I've seen numerous people say some time in natural sunlight will help stimulate a bowel movement. As long as it's a) warm enough and b) he has a place to get out of the sun if he wants, time in natural sunlight can't really be a bad thing.