/**
 * Limit the posts offered in the Post Reference dropdown to those with a particular term assigned
 * 
 * Edit 
 * 1. the slug of the post type in the if statement as required ('review' in this example) as well as the custom field slug 'wpcf-my-reference'
 * 2. the slug of the taxonomy ('colour' in this example)
 * 3. the slug(s) of the terms that should match ('red' in this example)
 */
  
function ts_filter_related_post_selector( $query )
{
    if ( defined('DOING_AJAX') && $_REQUEST['action'] === 'types_post_reference_field' && $_REQUEST['field_slug'] === 'wpcf-my-reference' && $query->query['post_type'][0] === 'review' ) { // edit
  
        $taxquery = array( 
            array(
                'taxonomy' => 'colour', // edit
                'field' => 'slug',
                'terms' => array('red'), // edit
                'operator' => 'IN'
                ) 
            );
  
        $query->set('tax_query', $taxquery);
    }
  
}
add_action('pre_get_posts', 'ts_filter_related_post_selector');