See the Game Guide "Tuning Your Machine" section on (I presume you are trying to run full-screen) re "Psychedelic" Screen.
Basically - some graphics cards
or I have found, some programmes! - will mess about with the 256 colour registers. Some boxes will happily let you swap in and out of the game from full screen with ALT+TAB, others will not. It depends on your graphics card and/or drivers and
also any third party programmes running at the same time as the game.
Mine was happy to run full screen until recently, apart from if I ran Microsoft Excel 95. But I recently de installed Norton Security, and installed Mailwasher and Avast!. These seem to do something with cutesy animating of their icons in the status bar tray (even though not needed if a full screen game is running), and that seems to mess with the 256 colour registers. Any program that does "pop ups" to tell you its updated its database (like some AV stuff) - is also a likely culprit, or which insists on ramming your game session to the back in minimised format to spam you with a windows desktop dialogue box can do this.
The cure for this is -
either find the programmes that are causing the clash, and make sure they are not running in the background (if you need to switch antivirus off of course - pull your network lead before each session!
),
or - the simple cure is to run in windowed mode. In Windowed mode, WinVFX graphics library maps the 256 colours to Windows compatible RGB so the 256 colour RAMDAC is not involved. I now run in 1280 by 1024 Windowed mode rather than full-screen mode, as it is easier than stopping the anti-virus!.
Cheers
Andy