void setup() {
  size(640, 360);
  background(51);
  noStroke();
  noLoop();
}

void draw() {
  
  drawCircles(width/2,  height/2,  300,  5);
  
  drawCircles(width/4,  height/4,  150,  8);
  
  // Kanete ki alles kliseis tis drawCircles() me alles times
  // gia na emfanistoyn ki alloi kykloi
  // gia paradeigma: drawCircles(width*0.75,  height*0.60,  150,  4);

} // telos draw

void drawCircles(float x, float y, float diam, float number)
{
  float xroma = 0;
  int i;
  float first_diam;
  
  first_diam = diam;

  fill(xroma);
  ellipse(x, y, diam, diam);

  for (i=1; i<=number-1; i++)
  {
    diam = diam - first_diam/number;
    xroma = xroma + 255/number;

    fill(xroma);
    ellipse(x, y, diam, diam);
  } // telos for
} // telos drawCircles