Categories
Categories let you tag Outlook objects (E-mail, Contacts, Appointments, Tasks) by a name and a color. The default Categories are simple: Blue, Green, Orange, Purple, Red and Yellow.

The built-in Categories may work for some, but it may be more useful to create your own custom Categories. Here are the steps.

5. Try It: Manage the Categories
Select an Event: Microsoft Excel 2013 Training.
Go to Event Details->Edit.
The Event should open.
Go to Event->More->Categorize.
Click on Manage Categories.

Keep going. This part is kinda fun...

Event->More->Categorize->Manage Categories