<?php

add_action('admin_head', 'orbisius_tutorial_7459_render_styles');

/**
 * This will override the Completed Order Status in WooCommerce - Orders
 * @return void
 * @see  https://orbisius.com/7459 
 */
function orbisius_tutorial_7459_render_styles() {
    ?>
    <style>
        .order-status.status-completed {
            color: #fff !important;
            background: red !important;
        }
    </style>
    <?php
}

Fuente: https://orbisius.com/blog/change-color-completed-order-status-woocommerce/