To change the Print Invoice button colour in WooCommerce order mail - By WebToffee (WooCommerce PDF Invoice, Packing Slips, Delivery Notes and Shipping labels)

functions.php

<?php //do not coopy this line of code
add_filter("wt_pklist_alter_style_for_email_button", "wt_pklist_alter_document_button_style_email", 10, 2);

function wt_pklist_alter_document_button_style_email($style_arr, $template_type) {
    $style_arr['background'] = '#ff0000'; //Change button background color
    $style_arr['border-color'] = '#cc0000'; //Change border color
    $style_arr['box-shadow'] = '0 1px 0 #990000'; //Change button shadow
    return $style_arr;
}