They change with lighting - interestingly they also change with mood (I would expect something to do with blood flow).

When I'm angry, or very excited or something similar, they apparently turn brilliant green.

I once came downstairs to let some folks into the house who had come to visit. My then g/f was upstairs finishing getting dressed as we had been engaged in - umm - mutually beneficial exercise.

I was like, "Sara's just getting changed, she'll be right down."

One of the new arrivals looked me straight in the eye, laughed and said, "You were just getting laid you bastard! Your eyes are bright green!" I'm stuttering trying to think of an excuse and she goes, "Don't bother - *I* know what they do from the last time *I* slept with you!"

Ooops. Busted.