float delta;

void setup()
{
  size(800, 800);
  background(220);

  colorMode(HSB);
  delta = 0.0;
  frameRate(10);
}

void draw()
{
  float  diam;
  float centerx, centery;
  
  diam = 30;
  centerx = width/2.0;
  centery = height/2.0;
  for (float y=diam/2.0; y<height; y+=diam) {
    for (float x=diam/2.0; x<width; x+=diam) {
      
      fill((dist(x, y, centerx, centery)+delta)%256, 255, 255);
      noStroke();
      ellipse(x, y, diam, diam);
    }
  }
  delta += 5;
}