I have a planet that I want to have a greenish atmosphere. I've heard in a few places that chlorine gas has the potential to make the atmosphere greenish, but also to kill humans. So, the question is, what percent of Earth's atmosphere (in nitrogen) can I replace with chlorine gas in my atmosphere without seriously damaging humans? Humans would have to be able to inhabit this planet without dying because of the gas.
Note - A less important side question, what color would the atmosphere look like if I added the maximum amount?