SOURCE

function setup(){
  createCanvas(500,500);
  background(200);
  rect();
}
function draw(){
  for(let y=0;y<500;y+=100){
    for(let x=0;x<500;x+=100){
        stroke(random(120),random(120),random(255));
        rect(x,y,50,50);
    }
  }
}