pewc_filter_global_categories_taxonomy.php

<?php
/**
 * Display global groups according to attribute or taxonomy
 */
function prefix_filter_global_categories_taxonomy( $args,	$group_id, $group, $rule ) {
  // Return the slug of the taxonomy you wish to use instead of product category
	return 'pa_colour';
}
add_filter( 'pewc_filter_global_categories_taxonomy','prefix_filter_global_categories_taxonomy', 10, 4 );