Could a planet have a green atmosphere?

We put this question to Dave Ansell...

Dave - Okay so I think it be difficult to get a green atmosphere in the same way as we have a green atmosphere on Earth, which is scattering and mostly at the blue end of the spectrum. You might be able get a slightly green tends to that if you're very very clever but I can't really see how you could do it. The only gas I can think of which is very definitely green is chlorine and that's an incredibly reactive gas similar to oxygen. So you'd have to kind of hypothesize some kind of life based on taking in sunlight and giving out chlorine rather than oxygen.

Chris - We have life that uses oxygen because oxygen was abundant. And it's an oxidizing agent. So it's not beyond the ken of reasonable grounds that chlorine could do that is it? As you can light a candle and put it in a vat of chlorine and the candle will burn...

Dave - Yeah. So it's not entirely implausible. I wouldn't want to visit it' because it would be a scary place to go!

Chris - Pretty horrible place! There you go Jeff! So you need planet "Cloros" or something similar! We think chlorine...

Jeff - Okay!