Split RGBW hex String into integers
I don't like using the String class because it can cause memory fragmentation. This would do it by just using C-style strings:

byte fromhex (const char * str)
  {
  char c = str [0] - '0';
  if (c &gt; 9)
    c -= 7;
  int result = c;
  c = str [1] - '0';
  if (c &gt; 9)
    c -= 7;
  return (result &lt;&lt; 4) | c;
  }

void setup ()
  {
  Serial.begin (115200);
  Serial.println ();

  const char * test = "#112233AB";

  byte redval   = fromhex (&amp; test [1]);
  byte greenval = fromhex (&amp; test [3]);
  byte blueval  = fromhex (&amp; test [5]);
  byte whiteval = fromhex (&amp; test [7]);

  Serial.println ((int) redval);
  Serial.println ((int) greenval);
  Serial.println ((int) blueval);
  Serial.println ((int) whiteval);

  }  // end of setup

void loop ()
  {
  // whatever   
  }  // end of loop


Output:

17
34
51
171


That does not do any validation (that the string is indeed 9 characters long, and contain hex characters) but I'll leave that for you to do. :)