int colCount;

void setup() {
  size(400, 400);
  background(255);
  rectMode(CENTER);
  
  colCount = height;
  colorMode(RGB, colCount);
}

void draw() {
  float x = random(width);
  float y = random(height);
  float s = mouseX;
  
  if (mousePressed) {
    fill(y, colCount, 0, colCount * 0.8);
  } 
  else {
    fill(colCount - y, colCount, 0, colCount * 0.8);
  }

  rect(x, y, s, s);
}