let amount = 0;

​

function setup() {

  createCanvas(400, 400);

  noStroke();

}

​

function draw() {

  if (amount > 1 || amount < 0) {

    step *= -1;

  }

  amount += step;

​

  let red = color(255, 0, 0);

  let blue = color(0, 255, 0);

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

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

  background(redBlueLerp);

  for (let i=0; i<windowWidth; i += 20){

    fill(blueRedLerp);

    rect(i, 0, 10, windowHeight);

  }

}