What kind of atmosphere would a planet need to alter the color of the sun and moon? I know this is rather broad, but I know that a certain gas in our atmosphere colors the sky blue, and gives the sun a yellow tint. So, what kind of gases, and how much of them, would be required to alter the coloration of the sun to red?
This is actually a common event right here on earth. You don't need to alter the actual chemistry of your atmosphere for this, you just need to have lots of particulates in the atmosphere. On earth this is usually caused by smoke from wildfires or volcanoes, or by dust storms.
If there's a reason you can't rely on particulates for this, then you'd need to have an atmosphere consisting of significantly heavier elements than ours does, which by extension would require MUCH higher pressures and temperatures, so if this is a planet that humans are supposed to be able to live and breathe on without safety equipment, I'm not sure that'll work for you.
in researching a similar question for my own stories, I found argon has the coloration I was looking for, while still being a common enough element in our own atmosphere to facilitate human habitation. it isn't really the atmosphere that has the color anyway. Like the depth of the ocean and the algae living there on you get green water Ie. much of the Atlantic ocean, and blue water, much of the pacific ocean, though there are exceptions to the rule. it's really the depth of the ocean(s) that give the color attributes the farther down the floor of the ocean is, the darker the color of the water over it. So to is the atmosphere. the air is clear, what we attribute as color is actually the light refraction of deep space through our atmosphere. this can be seen on the surface depending on the altitude of the land mass. those at or below sea level have more intense colors of sky whereas higher altitudes have paler colored skies. Hope this helps.