You can categorize an individual E-Mail or a complete E-Mail thread with a coloured label.
How to categorize an E-Mail with a label:
Select an E-Mail in the sidebar.
Click the Label icon in the display area.
Click on a label in the menu.
In order to remove the label, click on None in the Label menu.
Tip: You can also categorize multiple E-Mails at once or all the E-Mails in an E-Mail thread with a label.