In order keep configuration simple, please perform the following steps in Salesforce Classic (Booker25 is Lightning-Ready, you may afterwards use the app in SalesforceLightning as you please)
Reservation Statuses are used to define the status of a reservation, for example Optional or Definite reservations. Each status can have a different colour, and can or can not be editable once the status has been set. For example: Completed reservations can no longer be edited.
Create Reservation Status:
Go to the Settings tab in the Booker25 app
On the Related tab, scroll down to the Reservation Statuses list
Click New to create a new Reservation Status
Define a Name for the status
Define if the status should be the Default
Make sure it is Active in order to use it
Making the status Final means the reservation will no longer be editable once the status has been set. For example: a Completed status that should block the reservation from being editable
Create the colours for each Reservation Status
In the menu bar, click on Calendars
Click Go! next to All
Select the calendar for which you want to define colours
Scroll down to the Reservation Colors related list
Click New Reservation Color
Make sure to fill in the Reservation Status (which you just created) and Reservation Type (by default, this is Reservation - you still need to search for it in the lookup in order to show) fields. This combination will define the colour
You can either fill in the colours as shown in the screenshot, or fine tune the colours further using HEX codes
Make sure colours exist for all the Reservation Status / Reservation Type combinations in your org. Not doing so may cause unexpected results.