he says in the comments:

===

Hi,

File "symbols.png" is 10 by 10 grid of black and white icons. So there are totally 100 different icons. Each icon is 47 by 47 pixels. so total dimensions of "symbols.png" should be 470 by 470 pixels.

You can get file "symbols.png" by downloading sketch using usual "Download" link above, and then unzipping archive "ColorBlindness.jar".

===

Thanks for answering me !! : )
Actually, i would like to know if it's possible to create a code without the 'symbols.png', just drawing ellipses with the loops with a similary effect ... ?

Sure. Remove these lines:

1. PImage symbols;
2. symbols = loadImage("symbols.png");

Then change this line...

1.   if (brightness(symbols.get(xx, yy)) >= random(1)) {

...into something like this...

1.   if (random(1) >= 0.5) {

Thank You so much !!!

I would like to understand the whole code but this parts are a bit confused for me, can you explain me please ?
Sorry to disturb you again !

class CircleSet {
  ArrayList circles = new ArrayList();                 // Is it the way to draw the circles in arrays for many size ?
  ArrayList activeCircles = new ArrayList(); 

  CircleSet() {
  }

  Circle addRandomCircle() {                                                   
    int x = round(random(5, width-5));                                        // What means checkFit ?
    int y = round(random(5, height-5));
    return checkFit(x, y, 2) ? new Circle(x, y) : null;    
  }

  boolean checkFit(int x, int y, int r) {                                            
    return checkFit(x, y, r, null);
  }

  boolean checkFit(int x, int y, float r, Circle c) {
    for (int i=0; i<circles.size(); i++) {
      Circle circle = (Circle)circles.get(i);
      if (circle !=c) {
        if (dist(x, y, circle.centerX, circle.centerY) <= r + circle.radius + 1) {
          return false;
        }
      }
    }
    return true;
  }

  void growAll() {
    ArrayList oldActive = activeCircles;
    activeCircles = new ArrayList();
    for (int i=0; i<oldActive.size(); i++) {                                       //       it's the code to make the circles bigger ?
      Circle circle = (Circle)oldActive.get(i);
      boolean growSuccess = circle.grow();
      if (growSuccess) {
        activeCircles.add(circle);
      } else {
        addRandomCircle();
        addRandomCircle();
        addRandomCircle();       
      }
    }
    int count = 500;
    while (activeCircles.size() < 3 && (count--)>0) {                     //     what means those three last lines ?
      addRandomCircle();
    }