To customise colours on the appointment book, go to Settings > Appointment Book > Appointment Book
Select Edit
Scroll down to the Colours section where you can adjust the colours to your liking.
Appointments change colour after they are invoiced
Once an appointment has been invoiced, it is shown in the processed colour. To keep appointments in their normal colour instead:
Open the Appointment Book.
Select the settings cog at the bottom of the left sidebar.
Scroll to the bottom and untick Show Processed Colour.
This setting is saved per user, so it only changes how you see the appointment book. It does not change what other users see.