protected function adjustHsl($color, $idx, $amount)
{
$hsl = $this->toHSL($color[1], $color[2], $color[3]);
$hsl[$idx] += $amount;
$out = $this->toRGB($hsl[1], $hsl[2], $hsl[3]);
if (isset($color[4])) {
$out[4] = $color[4];
}
return $out;
}