One sci-fi staple that I've never come across a discussion of is the odd-colored alien atmosphere. Whether it's green, or purple, or orange, there are countless worlds in fiction with an unusual sky color - which is, of course, perfectly breathable by humans.

I'm curious to find out A) what sort of atmospheric compositions could reflect a uniformly non-blue color, and B) whether these atmospheres would be human-friendly. To make life easier for all of us, let's assume that the world in question is identical to Earth in all other respects, and orbits a likewise identical star.

Thanks in advance!