How to embed YouTube videos:

RGB Slider Example

import processing.serial.*;
Serial myPort; // declaration

// r01, g01, b01 are values from 0...1  
float r01;
float g01; 
float b01;
float boxw;
float boxx;

byte myByteArray[];

//=====================================================
void setup() {
  size (400, 350);
  boxw = width - 100;
  boxx = 50;
  
  r01 = 1.0;
  g01 = 0.8;
  b01 = 0.8;
  
  myPort = new Serial(this, Serial.list()[0], 9600);
  myByteArray = new byte[4];
}

//=====================================================
void draw() {

  // compute the colors from the RGB values
  float rCol = map(r01, 0, 1, 0, 254);
  float gCol = map(g01, 0, 1, 0, 254);
  float bCol = map(b01, 0, 1, 0, 254);
  background(rCol, gCol, bCol);

  float rW = map(r01, 0, 1, 0, boxw); 
  float gW = map(g01, 0, 1, 0, boxw); 
  float bW = map(b01, 0, 1, 0, boxw); 

  // draw black rectangles in background
  fill (0, 0, 0); 
  rect (boxx, 50,  boxw, 50);
  rect (boxx, 150, boxw, 50);
  rect (boxx, 250, boxw, 50);

  // draw RGB sliders
  fill (rCol, 0, 0); 
  rect (boxx, 50, rW, 50);

  fill (0, gCol, 0);
  rect (boxx, 150, gW, 50);

  fill (0, 0, bCol);
  rect (boxx, 250, bW, 50);

  // perform interaction which sets the RGB values
  if (mousePressed) {
    if ((mouseY > 50) && (mouseY < 100)) {
      r01 = constrain(map(mouseX-boxx, 0, boxw, 0, 1), 0, 1);
    } 
    else if ((mouseY > 150) && (mouseY < 200)) {
      g01 = constrain(map(mouseX-boxx, 0, boxw, 0, 1), 0, 1);
    } 
    else if ((mouseY > 250) && (mouseY < 300)) {
      b01 = constrain(map(mouseX-boxx, 0, boxw, 0, 1), 0, 1);
    }

    int rArd = (int)(rCol); // get integer versions of the floats..
    int gArd = (int)(gCol);
    int bArd = (int)(bCol);
    
    println(rArd);
    
    
    myPort.write (rArd); 
    myPort.write ('R');
    myPort.write (gArd);
    myPort.write ('G');
    myPort.write (bArd);
    myPort.write ('B'); 
    
  }
}

Arduino code:

int led0=3; int led1=5; int led2=6;
int rval=0; int gval=0; int bval=0;

char buff[]= "0000000000";

void setup() {
  Serial.begin(9600);
  pinMode(led0,OUTPUT);
  pinMode(led1,OUTPUT);
  pinMode(led2,OUTPUT);
}

void loop() {
  analogWrite(led0,bval);
  analogWrite(led1,gval);
  analogWrite(led2,rval);

  while (Serial.available()>0) {
    for (int i=0; i<10; i++) {
      buff[i]=buff[i+1];
    }
    buff[10]=Serial.read();
    if (buff[10]=='R') {
      rval=int(buff[9]);
    }
    if (buff[10]=='G') {
      gval=int(buff[9]);
    }
    if (buff[10]=='B') {
      bval=int(buff[9]);
    }

  }
  delay(10);
}

modified from code by Anthony Maddox.