We're using Microsoft Office Outlook 2007 and want to add a task marked with a category named Service to a user's task list and have it appear to that person in the same color for the Service category that everyone else sees. Why won't Outlook let us create a new category and apply it to items in a user's folder?
Category names and colors are part of the user's mailbox settings. As with rules to process incoming items, only the mailbox owner can change the category names and colors. Thus, if you add a task and click the Categories button, you'll see only the categories that the mailbox owner has defined; you won't be able to add others.
However, a partial solution to your problem exists. Create the task in your own Tasks folder and assign it the Service category. Then move the task to the user's folder. When the user looks at his or her tasks, the task you created will be marked with the Service category, it just won't have the special color that you specified for Service and won't be in the user's Master Category List. The user can then, of course, add the category to his or her Master Category List and assign the category a color.