One complaint I hear every now and again is that colors won't show in the Categories column in Outlook. The colors do show in an opened message or in the reading pane, but not in the message list. This is a bug in all recent versions of Outlook and it's easily fixed.
This happens when you move the Categories column to the left of the From field, or more accurately, immediately to the right of a field that uses an icon. If the field on the left is a text field, the colors will show.
Resetting the view will fix it (because it will move the category field to the far right); if you want the Categories field to the left of From, you can change the field on the left to use text instead of an icon format and the color block will display once again.
For example, when the Reminder field is an icon, you'll see a bell in the column. When it's set to On/Off, you see the word On when a reminder is set.
To make this change, switch to the View tab, select View Settings then Format Columns. Select the field right before Categories and choose one of the text options (Yes/No, True/False, or On/Off).