Turns a HEX color to a darker/lighter shade. If your needs are more intensive and you require more that just a function, you could check the phpColors repo: https://github.com/mexitek/phpColors

darkenlight.php

<?php
/**
 * Turns a HEX color to a darker/lighter shade.
 *
 * @param string $colorstr
 *   The color as a HEX string with a # prefix.
 * @param int $steps
 *   The number of steps to alter the color to. Both negavive and positive
 *   numbers are allowed.
 *
 * @return string
 *   The modified HEX string.
 */
function darkenlight($colorstr, $steps) {
        $colorstr = str_replace('#', '', $colorstr);
        $rhex = substr($colorstr, 0, 2);
        $ghex = substr($colorstr, 2, 2);
        $bhex = substr($colorstr, 4, 2);
        $r = hexdec($rhex);
        $g = hexdec($ghex);
        $b = hexdec($bhex);
        $r = max(0, min(255, $r + $steps));
        $g = max(0, min(255, $g + $steps));
        $b = max(0, min(255, $b + $steps));
        return '#' . dechex($r) . dechex($g) . dechex($b);
      };