I'm currently writing a script about planetary colonization. And I've got the idea of a planet with a kind of "colored stains" in its atmosphere but still suitable for life in both locations. I was looking for colored gases that could look like clouds to make that stain. But I was wondering what could be the cause for the gas to exist. During a discussion with some friends we arrived to two conclusions:
[The meteor one]
During the formation of the planet a meteor crashed, causing a deep crater. With the ages passing, the nature had regrown around the meteor. But minerals in the meteor cause a reaction with gas molecules in the atmosphere. It then creates the colored gas and due to his density it would be kept in the crater. So as Nature gets closer to the gas, it would start to mutate to suit the environment. And with years and years of mutation a symbiotic gas fed ecosystem would keep the gas production even if the meteor's minerals have dried up.
[The volcano one]
In the bowels of the planet, due to the planetary core composition, high pressure and rock composition, the colored gas is created and dissolved in the rocks. With the planet's volcano activity, the hotspots and more specifically those with high activity, the gas is liberated in the atmosphere causing the creation of the colored gas.
Our question is which one would be the most likely phenomenon (they surely need some adjustment) or if you have a better solution to this stained atmosphere we would gladly use it.