White Claw and Leeor,
You may have already thought about this, but something to consider is color grouping for different aspects of the game. For example in Outpost 2 they use 3 different colors for background terrain (grayish mud/dirt, orangeish/yellow sand, and the blacking dark rock). Eden structures are all white/light gray and Plymouth structures are all brownish. The structures stand out nicely from the surrounding terrain, except maybe Eden against the gray mud. They use 7 different colors to indicate player type which are all very vivid versions of the color that stick out from the background building and terrain colors.
I really enjoy the visuals in a game that has a thought out color pallete that allows different elements of the game to mesh and pop out well together. This might be a little counter to what we see in real life though, so maybe more of a preference thing.
In the pic Leeor posted, the bluish color of the tubes and buildings tends to blend in with the bluish terrain. The colony might pop a little nicer with more distinct color choices.
But, all that being said everything is looking great the way it is. With the graphics currently being added, I would be quite happy playing the game and enjoying the visuals.
Also, it is good to design the colors to work well for people with varying levels of color blindness. The hard part about this is people range in just having a little trouble to seeing green and red apart to only seeing in grayscale. I think someone mentioned having a little trouble seeing two colors apart in a different thread so they would actually be a great choice to look at the artwork regularly to make sure it makes sense.
One thing that can help in something like a GUI that uses color coordination is to also place a symbol in each color to add a second way to distinguish outside of color (not that OutpostHD needs this).
Even further off topic, it is a really good idea to enable multiple input devices and make the key bindings reconfigurable. This way people that have different disabilities and needs in regards to range of motion can reconfigure the game as needed. Maybe a mouse works better than a keyboard for some or even a gamepad. Perhaps they need keys on the keyboard mapped differently than is appropriate for what is considered normal typing. For a turn based game, this probably isn't as big a deal as in a real time game, but still a great feature.
Anyways, sorry if I digressed really far, just something on my mind and maybe better to think about now before the artwork gets too far. Although you all may have already covered it all.
Unfortunately, developing robust key bindings and proper graphics take a lot of time at the expense of other features that a small team of volunteers don't really have available to finish the project, so there is a balance I suppose.
-Brett