To add an event category navigate to Administration - Configuration - Event Categories
In the top right-hand corner of the screen, select 'Create New Category'.
On the following page, fill in a title for the event category, and background and font colours. When selecting background and font colours, clicking on the respective box will produce a drop down colour palette. Select your colour and the colour code will appear.
Select ‘Create Category’ in the lower right-hand corner of the page.
When adding an event to your calendar this is where the event category will display: