function allocate_color_from_hex($handle, $hex) {
	if (strlen($hex) == 4) $hex = '#' . $hex[1] . $hex[1] . $hex[2] . $hex[2] . $hex[3] . $hex[3];
 
	$red   = hexdec(substr($hex, 1, 2));
	$green = hexdec(substr($hex, 3, 2));
	$blue  = hexdec(substr($hex, 5, 2));
 
	return imagecolorallocate($handle, $red, $green, $blue);
}

$color = allocate_color_from_hex($picture, "#123456");