This question has been asked in similar forms on this site already with some good answers like here:
but I want to put a special emphasis on compatibility with life as we know it. So the question is: What possibilities exist to color the sky of an earth-like planet differently from our blue sky under the condition that life as we know it is supported?
The atmosphere consists of about 20 % O2, some N2 (for our dear plants!) and has a pressure of about 1 bar at sea-level
None of the atmospheric gases must be poisonous for (most) animals and plants
Photosynthesis must be possible
All constituents of the atmosphere must be chemically inert with respect to each other (e.g. large amounts of CH4 would combust with O2 at the earliest convenience and set the world on fire)
The color is seen by a human, so no fancy photoreceptors.
I'm open to suggestions about more conditions I may have overseen.
Now the possibilities of changing the color are to my knowledge:
Rayleigh scattering: Since the gases of our atmosphere possess resonance frequencies in the UV region, violet light is scattered more than red light. A quantitative deficiency in violet light (at least percieved by us) leads to blue being the dominant color. Are there gases with different resonance frequencies that could alter the scattering behaviour?
The sun: Emits a color spectrum according to it's temperature. By changing the temperature a different color can be obtained, but would this be possible in an extent that still satisfies the condition of permitting life? A strong red-shift would cause a UV-deficiency (photosynthesis) while a blue shift would increase the UV radiation to possibly hazardous levels for everyone.
The sun again: Next to the continuous spectrum defined by Planck's formula elements have strictly defined elemental lines. Could an enrichment of a certain element like Ca lead to a strong Ca-line that changes the sun's apparent color?
Particles in the atmosphere (like on Mars or even in Peking): In this case I am concerned about getting enough light to the surface to let plants live. Are there different colors possible or would it always be something red-ish?
Edit: I really don't know why this question has been flagged as duplicate. The linked question only covers one coloring mechanism and completely lacks the compatibility with life which is emphasis of my question as I clearly stated.