Adding a fourth colour to [app_schedule]


I was just wondering whether it is possible to add a fourth colour to the calendar. I want a separate colour that would then be used to differentiate between when the time has passed to book an appointment slot and when an appointment slot has been set to unavailable by the admin.

To show what I mean, I have attached a screenshot of the calendar. The place with a purple box is the slots that need to be a different colour. Is this possible?