int counter = 0;

void setup() {
  size(300, 300);
  smooth();
  noStroke();
  background(255);
  //frameRate(5);
}

void draw() {
  fill(random(255), random(255), random(255), mouseY);
  ellipse(random(width), random(height), 50, 50);
  
  counter++;
  
  if (counter > 200) {
    background(255);
    counter = 0;
  }
}