Example that shows how to modify the EE4 calendar's where parameters

ee_calendar_query_params.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter('FHEE__EED_Espresso_Calendar__get_calendar_events__query_params', 'my_custom_calendar_where_params', 10, 7 );
function my_custom_calendar_where_params( $where_and_order_by ) {
  $where_params = reset($where_and_order_by);
  $where_params['OR*category'] = array(
    'Event.Term_Taxonomy.Term.slug' => array( 'IN', array('red', 'blue', 'yellow', 'orange') ),
  );
  $where_and_order_by = array( $where_params, 'order_by'=>array( 'DTT_EVT_start'=>'ASC' ) );
  return $where_and_order_by;
}