Ah-ha! I've found it!
Sock this away for future reference: fiddling around with the "Flat Panel Scaling Options" in the nVidia control panel cleared it up.
- Use NVIDIA scaling -works
- Use NVIDIA scaling with fixed-aspect-ratio -colors correct, but bottom 1/3 of screen cut off
- Use my display's built-in scaling -works
- Do not scale -full screen shown, but the colors are a bit scrambled.
This game is in 320x200 resolution, right? I suspect that the software that handles scaling in the nVidia driver isn't designed to handle such a low resolution or doesn't handle such low resolutions as well in all cases, hence the squirrely behavior.