If you need to set some dates in calendar with different color (for example Holidays), then you can do this customization.
Example of changing color for 14th of February 2025.
Please add the CSS to your Theme Editor settings or in your external CSS file of your theme.
You can check how to add own CSS customization to CSS Theme Editor here: https://wpbookingcalendar.com/faq/how-edit-file-in-wp-menu/

Here's the code you need to add:

.datepick-days-cell.date_available.sql_date_2025-11-14 {
  --wpbc_cal-available-day-color: #4F11AA;
}

Where CSS class .sql_date_2025-11-14 define the DATE in format 2025-02-14.
And this variable --wpbc_cal-available-day-color: #4F11AA; define color of available date (in non legacy calendar skins).

Please do not forget to clear the browser cache before testing these changes.