The color of all atmospheres, by default is blue.
A common misconception is that it is oxygen that makes our sky blue. This is not the case. Oxygen is a colorless gas. All colorless gasses will have Rayleigh Scattering, where shorter wavelengths of light get scattered out, making the light source appear redder and the rest of the sky appear bluer. This is the same physics that makes sunsets reddish.
The sky of all of the gas giants would be bluish, as shown in these artworks of the gas giants:
Jupiter, with hypothetical alien life forms, from the old TV show Cosmos.
Saturn’s cloud tops with the Cassini robotic spacecraft burning up in the upper atmosphere, artwork made to commemorate the end of Cassini’s mission.
I can’t really find any good artwork of Neptune and Uranus’ sky.
You can see the blue rayleigh scattering from space in these images of Jupiter and Saturn:
Both the subtle blue haze at the edge of the planet, and the darker blue haze filling up the polar cap of Jupiter, are the result of Rayleigh scattering haze.
(Look for the subtle blue haze around the edge of Saturn)
You can even see blue haze on Mars’ atmosphere:
So why do all the planetary skies we have pictures of not look blue?
Venus seen by the Venera spacecraft. Note the sickly yellow sky
Mars with the Sojourner Rover seen from the Pathfinder lander. Note the butterscotch sky.
Titan’s surface as seen by the Huygens lander. Note the orange sky.
Then there’s the black sky of the Moon, which is of course just due to the lack of an atmosphere.
So why am I saying the default atmosphere is blue? All the planets we have pictures from the surface of seem to disagree! Don’t worry I’m not insane. The reason is it’s not gas making these skies different colors.
Venus has clouds of sulfur dioxide. Clouds are not actually made of gas, they’re an aerosol (air solution) of droplets of liquid suspended in a gas. The clouds are so thick that the faint yellowish tint of the clouds (which from space are plain white) gets amplified to a deep sickly yellow color. We have no pictures of the sky above the cloudtops, but if we did they would probably look something like this:
Mars’s atmosphere is so thin that the blue part is only visible during sunset or sunrise when the light has to travel through a lot of the atmosphere to reach the observer.
But during the day, the sky is dominated by reddish brown dust, which is the same color as the surface.
Titan’s atmosphere meanwhile is full of organic photochemical smog, which is suspended in the atmosphere.
Other layers of Titan’s atmosphere are blue.
There are a few ways an atmosphere alone can look something other than blue. If the atmosphere is made of a tinted gas like chlorine, it might have a tinted sky. Alternatively, if the planet orbits a star that is cooler and redder than our sun, the sky would have sunset tones even at noon. Similarly, a cloudless planet with a very thick atmosphere around a sunlike star would have sunset tones at noon because of the extra air mass the light has to pass through.
Neptune has white clouds of ammonia, but most of them are deep in an atmosphere that is blue tinted not just by rayleigh scattering but by absorption of red light by methane.
Above the cloud tops, the sky would be dark blue (the atmosphere is fairly thin at that altitude, and the planet is under dim lighting conditions). The Sun would be slightly bluish as well due to methane absorption.