You can categorize an individual E-Mail or a complete E-Mail conversation with a colored label.
How to categorize an E-Mail with a colored label:
Select an E-Mail.
Click the Set color icon in the toolbar. Select a color from the menu.
You can also use the Set color icon in the detail view.
In order to remove the label, click on None in the Set color menu.
Tip: You can also mark multiple E-Mails at once with a colored label.
Also see
Parent topic Organizing E-Mails