White's Illusion

When I'm not walking along the beach at Aberdyfi, I like to code in p5js. This is an example of White's Illusion. The two squares are the same colour. Below is the code to make it yourself.

  function setup() {
  createCanvas(400, 400);
  noStroke();
}

function draw() {
  background(0);
  
  fill('blue');
  rect(250,50, 100,100);
  fill(255);
  rect(0,200,400,200);
  fill('blue');
  rect(50, 250, 100, 100);
  
  for (let i=0; i < 210; i+=20){
    fill(255);
    rect(0,i, 400, 10);
    }
  
  for (let i = 210; i< 400; i+=20){
    fill(0);
    rect(0,i, 400, 10);
  }
}