var invert = true;

​

function setup() { 

  createCanvas(windowWidth, windowHeight);

  noStroke();

} 

​

function draw() { 

  background(invert ? 0: 255);

  sz+=0.25;

  fill(invert ? 255: 0);

  ellipse(0, 0, sz, sz);

}

​

// Invert black and white

function mousePressed(){

 invert = !invert; 

}

​

​