I was watching Lord of the Rings extras a few days ago(yeah, still on my sick leave) and they told about this digital color grading they used in many scenes to create a specific mood. So I got this idea that the similar method could be used in Xenonauts to ephasise different moods in different tilesets and night time.
I suppose the implementation should be really easy, just one alphatexture rendered behind the UI texture. Please add one, just for us modders atleast!
Here's a visual concept what I was thinking...
Middle east, a bit of sand in the air:
Arctic, freezing blue mist:
Night time, blue hue and dark corners:
These are just really quick few minutes examples, and I overdid the effect on purose to make it stand out, but I think this could really add to the mood and graphical fidelity of the game and to further differentiate tilesets from each other.