If you migrate to Office 365, Outlook.com, or Exchange server, or import contacts, appointments, or mail that is categorized but the category is not on the master list Outlook has a feature that will look for categories assigned to your Contacts, Appointments, and Mail and add them to your master categories list. The colors will be randomly assigned (not totally random: Outlook chooses the next color on the list).
Do this by right clicking on the top level folder (where Outlook Today is, or for secondary accounts in your profile, on the top level of the data file) and choose Data File Properties from the menu.
It should be the last command, as seen in the screenshot, but may not be the very last item if you have add-ins installed that add commands to this menu.
In the Properties dialog, click the button to Upgrade to Color Categories. This will convert the categories and add any not already on your master color category list, to it.
If you have more than one account in your profile that uses an existing data file, you need repeat these steps for each account.
To share color categories with other users, see the Tools below.
Tools in the Spotlight
Category Manager allows sharing of color categories; it adds a sidebar to the folder view as well as opened items, in which you can group your categories and assign them lightning fast; and it adds a reminder, which optionally prompts you if you forget to assign a category. Version 3
ReliefJet Essentials is a set of 166 utilities for Outlook, Office 365 and Exchange Server. It can backup, import, export and convert data into different formats. Perform personalized mass mailings. Save, remove and optimize attachments. Find and remove duplicate messages, contacts, appointments, meetings, and other items. Retrieve statistics on email usage and get reports on Outlook data and settings. You can run the utilities directly from Outlook, automatically in rules, via standalone app, or from the command line.
CodeTwo CatMan allows sharing of Outlook categories with other users on the local network. Centralized management using a shared configuration file. Version 3.0.1
Exchange Category Manager is a plug-in for Exchange Server and Office 365. It enables you to manage the Outlook categories and colors that are available in Outlook. Define an unlimited number of groups with different users. Each group can have it's own set of categories and colors to choose from. You can add categories to the existing category list that a user has in Outlook or replace everything with the predefined list. As it integrates with Exchange server directly, the users do not have to do anything, their category list is automatically managed. No software has to be installed on the client pc's.