Sometimes the categories are no longer valid and appear as white squares.

Also, each category cannot be assigned unless it is re-created.

If there are a lot of categories, then this process can take sometime.

Microsoft provided a solution to resolve this tedious behavior, but it is one of those hidden features.

1. Ensure that Outlook is open and the Mail tab is selected.

2. Right-click on the account that contains the categories and click on the Data Properties... item.

Click on Data Properties...

3. Click on the Upgrade to Color Categories... button to continue the process.

Click on Upgrade to Color Categories... button

4. A dialog prompt appears that informs