Oddly, I cannot find an answer to this question in the manual, by searching my hard drive, here on DVXUser, on Apple's discussion boards, or on Creative Cow. Does anyone know the answer?
Go to your Color project in the Finder, right click on it, select Show Package Contents and look in the "Archive" folder and if you have any backed-up versions of your project that's where they'll be.
The "Archive" folder in the project package includes files created when using the Save and Archive command in Color, and these are different than autosave files. A new archive file is also created when a project is manually saved, but an archive is not created when an autosave occurs. I think that Zak's initial response was correct--autosave seems to update the actual project file, but does not create an archive file or an attic of autosave project files.