For example when I read pixel color from an image, why I should use

$r = round(round($rgb['red'] / 0x33) * 0x33);
$g = round(round($rgb['green'] / 0x33) * 0x33);
$b = round(round($rgb['blue'] / 0x33) * 0x33);

I'm trying to feagure it out the right hexa color of rgb when breaking a part pixel by pixel and image.