Reorder the visible attributes on the frontend.

reorder-attributes.php

<?php
/**
 * Reorder the visible attributes on the frontend.
 */

add_filter( 'woocommerce_product_get_attributes', function( $old_attributes, $obj ) {

	$order = [ 'pa_colour', 'pa_size' ];

	$new_attributes = [];

	// Loop through our order items.
	foreach ( $order as $key ) {
		if ( isset( $old_attributes[ $key ] ) ) {
			
			// Add the attribute to our new attribute array.
			$new_attributes[ $key ] = $old_attributes[ $key ];
			
			// Remove the item from the old attribute array.
			unset( $old_attributes[ $key ] );
		}
	}

	// Merge any possible leftover attributes into the new attribute array.
	$new_attributes = array_merge( $new_attributes, $old_attributes );

	return $new_attributes;
}, 10, 2 );