Color categories allow you to identify associated items in Outlook, so you can quickly organize and track items, you can also assign more than one color category to items.
You can choose from the set of the default items or create your own.
To assign a color category to your message right from the message list, simply, Right-click the message, select categorize, then select the color category from the list
Note: you can perform this actions multiple times to assign a second or third color category.
Note: The Categorize drop-down displays the ten most recently used categories. If you have additional categories, you’ll need to select All Categories to see the rest of the list.