public function darken(Color $color, $amount = null)
    {
        $current = $color->lightness;

        $amount = ($amount === null) ? self::DEFAULT_ADJUST : $amount ;

        $amount = intval($amount) / 100;

        $new = $current - $amount;

        $new = ($new < 0) ? 0 : $new;

        $new = ($new > 1) ? 1 : $new;

        $color->lightness = floatval($new);

        return $color;
    }