Add anchor text to php

You can use the_title filter. Just remember to remove it after wp_list_pages() otherwise it will effect everywhere on the site like in menu, main title, search results etc.

Example:-

add_filter('the_title', 'my_custom_title');
wp_list_pages();
remove_filter('the_title', 'my_custom_title');

function my_custom_title($current_title) {
    if ($current_title == 'blue') {
        $current_title = $current_title . ' widget';
    }

    return $current_title;
}