Jeff Frey provided a tip in this ResExcellence article on how to add additional highlight choices to the set provided with OS X.
The highlight choices are stored in a plist file buried in your System folder, and this file can be edited (most easily) in OS 9.0, and additional colors added by simply specifying RGB color values and a name.
Check out ResExcellence for the details!
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Mac OS X Hints
http://hints.macworld.com/article.php?story=20010209095328440