I think Kirk is underselling it slightly - if you're using Mac OS X 10.4 (Tiger), you can use PM's Search function, accessible from the Edit menu. This is based on the Spotlight search built-in to the operating system. It allows you to search your entire computer using many different IPTC and EXIF attributes (although you can't search for colour classes), and have the results displayed within Photo Mechanic.
Kirk, as a suggestion, it would be useful to be able to restrict this search to one folder (as you can from the spotlight interface).
Gareth.