So besides basic colors, we have in the calendar that are based on the guest status.
We also have the option to let you do custom colors based on channel or completely custom.
If you want that for example all reservations from Booking.com to be blue you need to do the next steps:
Settings > Channels > Edit desired Channel
After this please go to the calendar and in the filters section change reservation color to be by the channel:
Now you can click on any reservation and adjust its color manually if there is a need for that:
You can always return back to the original calendar color by going in the Filter > Color > By Guest Status.