Colour labels are a visual way of categorising your jobs, and are visible from all calendar views, and the technician app.
To assign a colour label
Go to either a new/existing job
Go to "Color label"
Select a colour
Save job
To add a new colour label
Go to either a new/existing job, or Setup > Jobs > Colors
Click New colour label
Enter the name in the "Label" field
Select the colour, either by using the selector, or using the HEX / RGBA values