In the Group Policy Editor (with the Outlook 2007 or greater ADM files installed), look under Miscellaneous. You can use Group policy to push Categories out to users, merging new categories with their existing categories.
This will assign random colors to the categories assigned to items in your data file. If you imported from an old data file, right click on the root folder (Outlook Today), choose Properties and click the button to Upgrade to color categories.
If you are setting up a new system and using your old pst the color categories will be available on the new system. Outlook does not use the registry for a master category list they are stored in the default message store (mailbox or pst).
