blue sunset in Mars

It's not possible currentlyIt's not possible currently, due we doesn't have a good model for coloring of atmosphere.

I have an experimental branch, https://code.launchpad.net/~georg-zotti/stellarium/gz_AtmosphereTweaks, where I allow playing with the parameters of the Preetham skylight model. However, this is stuck a bit in favor of more important tasks. You have to have the paper in front of you to understand the parameters. In principle, it should allow you to tweak the main atmosphere colors as long as they fit the general model used there. I could create some fancy "alien planet movie sets" with red sky and green solar highlight or so...

You could try to just set higher elevation (13.000m?) for a more transparent atmosphere, but then you will see the limits of the standard model with a super-bright twilight horizon.

Also here, we are open for hints on (existing, approved, published, available) models for parameterizing alien atmospheres!

http://elib.dlr.de/86477/1/Collienne_GI_VRAR_2013.pdf
https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky

Thanks. New blueprint: https://blueprints.launchpad.net/stellarium/+spec/atmosphere-model

But don't hold your breath. https://answers.launchpad.net/stellarium/+faq/2913. Better start coding yourself :-)