Since there aren't many written sources for sharing categories, I thought it might be useful to discuss a bit on this topic and how can this be achieved in Outlook 2010.
As a background information for versions prior to Outlook 2007 category information was stored in the local registry. There is a binary key, called MasterList, located under HKCU\Software\Microsoft\Office\x.0\Outlook\Categories that contains the information on the categories.
The key will be written only if you customize the Master List.
Since Outlook 2007 the information on categories is stored on the mailbox (more specifically in a Mapi property on the item IPM.Configuration.CategoryList within the Calendar, called PR_ROAMING_XMLSTREAM).
When you will upgrade users to Outlook 2007/2010 from Outlook 2003, each user's categories and flags are still available. Flags, reminders, and labels will be rolled into categories for Outlook 2007/2010. Outlook will migrate category and flag information into the new categories structure.
• Quick flags: When a user starts Outlook 2007/2010 for the first time, all existing quick flags migrate to categories. The same color is used, but date information is not retained. If an item has a reminder flag, the reminder is migrated separately.
• Calendar labels: Labels also migrate when Outlook starts and are added to the user's categories list.
• Other categories: Categories for non-mail items (such as tasks) migrate to Outlook 2007/2010 categories and a random color will be assigned to each category
Color categoryselections are stored in the users default mailbox storage location, which should be the mailbox on the server. This means categories now roam with the user. However, you may need to right click on the root folder, choose Properties and click the button to Upgrade to color categories. This will assign random colors to the categories.
Outlook 2007/2010 will no longer use the registry for a master category list. Instead, they are stored in the default message store (mailbox or pst). For example, if you are setting up a new system and using your old pst, the color categories will be available on the new system.
Although categories are not meant to be shared you might encounter scenarios in which an administrator for a shared mailbox would like to have the color categories visible for all the other users that access the shared mailbox.
The expected behavior is to only see the categories but not the colors.
However, if you want to make also the colors visible for other users there are two methods to achieve this:
1. Add the shared mailbox as an additional mailbox and provide Reviewer permissions on the Calendar.
This is required as the Category list is stored on the Calendar.
This is how you are seeing the categories for the additional mailbox
2. Add the shared mailbox as an additional Exchange Account.
In this way you will have two category lists, one for each Exchange Account.
For this you need know the credentials of the shared mailbox (unless you have Full Mailbox Access) and have at least Reviewer Permissions on the Calendar, like in method 1.
Note: Adding the additional Exchange Account with delegate access is considered unsupported.
This is how you are seeing the categories for the additional Exchange Account
This is how you are seeing the categories for your own account
For all the methods above, the already categorized items from the shared mailbox will have the color category visible.
To summarize this, there are several options for sharing and managing color categories in Outlook 2010:
1. Adding the shared mailbox as an additional mailbox and provide Reviewer permissions on the Calendar.
2. Adding the shared mailbox as an additional Exchange Account
Each of them have their own advantages and disadvantages, so please review the options and adapt them to your own environment, accordingly.
Please feel free to comment, raise questions or add extra information on the topic.