How could a habitable planet have a green atmosphere?

How could a planet which humans could land and survive on with no space suits or other help have a green atmosphere? The only requirement is that humans landing here would have to be able to breathe the atmosphere, and potentially live on the planet for their entire natural lives.

Note - I would like to know the percentage of different gases for your answer. For example, if you replace 20% of the atmosphere (in H) and replace it with imagineum then your atmosphere would appear green.

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How could a habitable planet have a green atmosphere?

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