wangdu_public_api/config/captcha.php

<?php
/**
 * Created by PhpStorm.
 * User: admin
 * Date: 2018/12/21
 * Time: 16:26
 */
return [
    'default'   => [
        'length'    => 4,
        'width'     => 240,
        'height'    => 72,
        'quality'   => 99,
        'lines'     => 6,
        'bgImage'   => false,
        'bgColor'   => '#ecf2f4',
        'fontColors'=> ['#2c3e50'],//, '#c0392b', '#16a085', '#c0392b', '#8e44ad', '#303f9f', '#f57c00', '#795548'
        'contrast'  => -5,
    ],
    // ...
];