I imagine it depends on the chemical makeup of their atmospheres.
And the 'color' of the star nearby and the distance to that star, and the depth and density of the planetary atmosphere, which would be functions of the composition and planetary mass (local gravity).
Our atmosphere appears blue, or yellow, or red, or white, or grey depending on time of day, angle of the sun, clouds or clear, dust (particulates) in the air. Our sun is a 'yellow' star, but it could have easily been a red star or blue star, and we would have a different environment, either too cold or too hot. The atmosphere on Venus would look very different.
Edit/update: I'd add temperature of the atmosphere, which be included in the state of the components as mentioned by sophiecentaur.
And the state of the components (e.g. are there droplets, crystals or dust ) in the atmosphere.
Separate names with a comma.