This can be done by clicking on the engineer listed on the right hand side of your calendar, on the website
If you are using the software as a company with multiple users then it is important to be able to differentiate between those users when creating calendar appointments. This will allow you to quickly recognise the workload for each user and determine their availability.
First, navigate to the calendar page. On the right hand side you should see a list of the company's users and engineers. By clicking on one of the engineers a colour picker will pop-up like in the image below.
Select a colour for the engineer and then click off the box to close it. The colour of the engineer and all their related appointments on the calendar will be updated to the selected colour.
Repeat these steps for each of your company's users.