Sorry for my quite terrible delay in replying. Yes, Teudogar uses a very, very old-fashioned 8 bit 256 colour palettized graphics mode, and in some constellations Windows messes up the colour palette.
I'm currently working on porting Teudogar to 16 or 32 bit non-palettized truecolor mode, and will make free updates available when done. But there are some things that will take a bit more work, including that the day/night/light system is based on palette modification, and thus will have to be rewritten to use a different method under truecolor.
Meanwhile, in order to avoid the colour problem with the present Teudogar version, you can close all other applications (particularly Windows Explorer in Thumbnail Preview Mode or Windows Media Player) before running Teudogar; also, close all widgets, clocks, tool bars, or whatever else is on your desktop that might get updated (if anything gets updated and output anywhere, Windows (esp Vista or 7) will tend to mess up Teudogar's palette (ignoring that, with Teudogar being in exclusive full screen mode, there's nothing to output and thus no need to mess with the palette; but, well, that's Microsoft). You might even hide the taskbar. With really nothing else left to output or to even consider for output but Teudogar, Windows should leave the colours alone.
Second, whenever the problem occurs, you can fix it by briefly pressing ALT+TAB, which will return you to the Windows Desktop, and then pressing ALT+TAB again, which will return you to Teudogar, which will then reload the graphics and the colour palette, thereby fixing any messed up colours.
I hope these suggestions help, at least until I finally release an update.