let amount = 0;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {  

  if (amount > 1 || amount < 0) {

    step *= -1;

  }

  amount += step;

​

  let red = color(255, 0, 0);

  let blue = color(0, 0, 255);

  let redBlueLerp = lerpColor(red, blue, amount);

  let blueRedLerp = lerpColor(blue, red, amount);

  background(blueRedLerp);

  noStroke();

  fill(redBlueLerp);

  rect(width/4, height/4, 200, 200);

}