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Here's some rough WIP pixel art of the planet I'm trying to create just to give you an idea of what I'm talking about color wise. I'm referencing the dark blue that you see at the poles of the planet.

enter image description here .

This planet is going to be similar to Neptune in a lot of ways, a little smaller, about twice as massive, and also in the cold outer solar system, but I'm not sure if methane would be able to create the deep, dark blue color in the atmosphere that I'm looking for. Maybe nitrogen or oxygen or some compound with those? Or some completely different chemical?

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    $\begingroup$ This other question is not a duplicate, but you should find it very, very helpful. $\endgroup$
    – JBH
    Dec 29, 2018 at 23:03

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Tiny swirling methane crystals.

https://www.universetoday.com/21591/color-of-neptune/

neptune

https://solarsystem.nasa.gov/planets/neptune/in-depth/

Neptune is really blue.

Neptune's atmosphere is made up mostly of hydrogen and helium with just a little bit of methane. Neptune's neighbor Uranus is a blue-green color due to such atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.

I can think of two ways to make something like Neptune blue. One is more methane, that absorbs red and reflects blue. The other is Rayleigh scattering. The shorter the wavelength of light, the more likely it is to bounce off of something and keep going in a different direction. That is why the sky is blue - short wavelengths like blue bounce off of stuff in the sky, possibly back down to our eyes while longer wavelengths get absorbed or keep on going. That is why colors drop out with ocean depth - first reds, then greens, with deep blue (and presumably ultraviolet) the last light to disappear - because these rays have bounced and scattered their way downward as other frequencies were absorbed.

If your atmosphere were very full of tiny swirling methane crystals they could serve double duty - absorbing reds and scattering blue and violet back out. I bet that is why Neptune is so blue and I bet that is the secret of Uranus' newly discovered blue ring.

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