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!

I'm not sure you can draw any definite conclusions about the human-friendliness of various sky colors. If the illumination source is the same, at least that variable... doesn't vary. Dusts, droplets, solutes, vapors, could make some difference and be of no health consequence.

Orion's Arm has a little about alien atmosphere colors: The Sky on Alien Worlds

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Interesting color article from Space.com:

We had a very long thread on this topic some time ago, but for the life of me I can't find it. Let's see if I can summarize.
Load the sky with coloured particulates, and you can have any colour you want.
If your atmosphere is composed of colourless gases and minimal particulates, then its colour will come from Rayleigh scattering. That gives a sequence of sky colours as atmospheric density increases:
Blue horizon, black or dark blue zenith (thinner than Earth's atmosphere)
Blue zenith, pale or white horizon (like Earth)
Horizon whiteness spreading towards the zenith, and "closing" to give a completely white sky somewhere around 10 atmospheres.
Beyond that, the sky progresses through a set of sunset colours: pale yellow, orange, red, dark red, utter darkness. (The "Belt of Venus", which rises in the east as the sun sets, gives an impression of what these shades might be like.)
There's also a possibility of a very delicate pale green in the transition from white to pale yellow: some reliable observers say they've detected such a shade in the Belt of Venus, but I never have. These dense-atmosphere sky colours would be very dependent on the height of the sun: you might have a pale yellow sky at mid-day which turned orange by mid-afternoon, for instance.

The trouble with the exotic sky colours is they all require atmospheres denser than 10 atmospheres. An Earth-type mixture of nitrogen and oxygen would be unsafe to breathe at that pressure: you'd either suffer oxygen toxicity or nitrogen narcosis or both.

Grant Hutchison

Scientists have theorized that before Earth's atmosphere had a lot of oxygen, that the sky was brownish or tawny in color, a result of a reducing environment.

The only condition I could imagine a differently-colored atmosphere being breathable would be around a red dwarf, in which there would be much less blue light for any atmosphere to scatter.

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