I know this has been asked before, but I'm new to Arduino and Processing, and I'm trying to take this step by step.

I would like to pass an RGB value from processing, into Arduino via the Serial.Write/Read.

I have my Processing as:

void draw(){

  //orange colour in RGB values
  color _rgbColor = color(255, 40, 0);

  //convert colour into hex
  String _hexColor = hex(_rgbColor, 6);

  //convert colour into binary
  String _binaryColor = binary(_rgbColor);

  //pass colour to port

I assume I can just pass the HEX value straight across, right?

First problem I get is that I can't debug what my Arduino device is getting (when I run the Processing program, the Serial Monitor wont work. Seems like I can only use one or the other). I can print the myPort.read within my Processing after the delay, and I get:


So I'm not quite sure what I'm doing wrong there.

On the Arduino side, I'm using Serial.read() to retrieve the Serial data, which I assume will all be broken down into bytes, so I need to then convert it into a HEX formate, right?

----- Update -----

I now know that I need to break down my color variable into a char-message. I'm guessing I can do something like this then, for example: And then on the other side (Arduino), I need to rebuild the message, right?

if(mousePressed == true){



My Arduino script is as follows, is this an okay route to go down?

String message;
void loop() {

  int _size = Serial.available();

  if(_size > 0){

    char value = Serial.read();
    message += value;

    //full message received (255,255,255)
    if(message.length() == 11){
      String _red = message.substring(0,3);
      String _green = message.substring(4,7);
      String _blue = message.substring(8,11);
      Serial.println("R:" + _red + " G:" + _green + " B:" + _blue);

      //apply to light system
      setColour(_red.toInt(), _green.toInt(), _blue.toInt());

      //clear buffer
      message = "";
  • You should digest this first: hackingmajenkoblog.wordpress.com/2016/02/01/…
    – Majenko
    Commented Dec 16, 2016 at 18:41
  • 1
    Also you should understand exactly how the UART protocol (RS-232) works.
    – Majenko
    Commented Dec 16, 2016 at 18:47
  • Surely someone, somewhere has already made a defined script for this? To simply pass a colour variable to the Serial? Commented Dec 16, 2016 at 19:52
  • 1
    Many people have, yes. It is a simple enough operation once you understand how a UART works that it becomes a non-entity and you don't need to really worry about finding special code for it.
    – Majenko
    Commented Dec 16, 2016 at 19:53
  • The simplest way is to print the three colour values as a comma-separated line, maybe with a prefix character to indicate the start (C23,49,119\n for example). Then it is a case of reading that string and interpreting it properly.
    – Majenko
    Commented Dec 16, 2016 at 19:54

2 Answers 2


Add print(_hexColor); to you processing sketch. The value it's sending should appear in the Processing Console (at the bottom)


I know I'm late, but actually got a working solution, when I was working out a solution for this.

Code in Processing assuming variable myPort is handled in setup():

void draw() {
    int r = 255;
    int g = 123;
    int b = 2;
    myPort.write('S'); // tell the arduino to do something with leds

Code for Arduino loop:

void loop() {
  int r;
  int g;
  int b;
  if(Serial.available()) {   
    if(Serial.read() == 'S') {
      r = Serial.read();
      g = Serial.read();
      b = Serial.read();
    delay(10); // some recovery time, not sure if nessecary

    // do something with rgb values

This works for me with every possible RGB range. It's not the most elegant solution, but by far better then converting between strings/chars and ints with different bit counts on different devices.

  • 002 is an octal number. In this case it's still 2, but 020 would be 16. Don't use leading zeros, unless you actually want octal numbers.
    – gre_gor
    Commented Sep 4, 2017 at 18:46
  • you're right, I wrote this on the fly!
    – Cronay
    Commented Sep 5, 2017 at 20:03

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