Code for Changing Item Quantity Color in WooCommerce Invoice document - By WebToffee ( WooCommerce PDF Invoice, Packing Slips, Delivery Notes and Shipping labels (free) )

function.php

<?php> //Do not copy this line of code
add_filter('wf_pklist_alter_item_quantiy', 'wt_alter_invoice_quantiy', 10 , 5);
function wt_alter_invoice_quantiy($item_quantity, $template_type, $_product, $item, $order)
{   
    if( 'invoice' === $template_type )
    {
        if( $item_quantity>1 )
        {
            $item_quantity = '<span style="color:red;">'.$item_quantity.'</span>';
        }
    }
    return $item_quantity;
}