You can categorise an individual email or a complete email thread with a coloured label.
How to categorise an email with a label:
Select an email in the side bar.
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 categorise multiple email messages at once or all the email messages in an email thread with a label.