You're missing the parameter name from the form data:
NSString *hexString = [NSString stringWithFormat:@"%02X%02X%02X", red, green, blue];
It should be
NSString *hexString = [NSString stringWithFormat:@"color=%02X%02X%02X", red, green, blue];
and then retrieve it server-side like this
$color = $_POST['color'];
Alternatively, if you don't want to specify a parameter name, you can get the full request body server-side like this:
$requestBody = file_get_contents('php://input'); // will be the hex string