Wow, never seen it corrupt the graphics like that before. I can see why you'd interpret that as a palletised video mode, but I checked and there's more than 256 colors in the gradients of that PNG you've posted.
The only thing I can think of right now that might cause this is really messed up Color Management settings, but that would be screwing up all kinds of things. If you hit the windows key and go to the Color Management control panel, there's a master switch labelled Use My Settings For This Device which toggles color correction on or off for a particular display. Does anything change if you toggle it off and relaunch the game?
I've tried a number of different color management settings, the monitor OSD, and everything I can think of, it's actually an older Dell square monitor that I am using temporarily (stupid cat shorted out TWO wide-screens in one week marking them as territory) and I'm sure it will be fine once I go back to a normal display. Might try switching cables around, but good enough to know it's something unusual ... maybe a satanic ritual is called for.... anyways, love the game, and thanks for the reply!