I have this old game that was made for Windows 95, so in XP I run it in compatibility mode (640x480, 256 colours). It runs fine. However, when I close it and Windows switches back to 32-bit colour, some colours don't look right. For example, the blue that is displayed around selected text now is a very bright light blue. A reboot fixes this, but is there any other way?
One possibility is to use the free VirtualBox (or similar). Run it under a virtual machine (it shouldn't have performance problems if it is expecting a mid-nineties machine). After running it, you can quickly switch back to a saved state on the machine (or just switch back to the unvirtualised desktop).
It does use quite a bit of memory, and it probably isn't worth it unless you are going to use VirtualBox for other reasons (it can be quite handy for some).
(Disclosure: I currently work for the company that produces VirtualBox, but I don't work on it.)