Use the Universal Access preference pane to switch your desktop to grey scale. It also allows you to zoom in on any area of the screen and even invert the color scheme to white on black. Makes a nifty demo, too.

I've also played with the various third party UI themes ([link|http://www.resexcellence.com|http://www.resexcellence.com] is a good source of information.) There are some very nice ones along with tools to make changing them out very easy.

The downside, of course, is that these third party themes are not officially supported by Apple and the usual warnings and disclaimers apply.