You can categorise an individual E-Mail or a complete E-Mail conversation with a coloured label.
How to categorise an E-Mail with a coloured label:
Select an E-Mail.
Click the Set colour icon in the tool bar.
Select a colour from the menu.
In order to remove the label, click on None in the Set colour menu.
Tip: You can also mark multiple E-Mail messages at once with a coloured label.