let startColor = 255;

let speedAnimate = 1;

function setup() {

  createCanvas(400, 400);

  noStroke();

}

​

function draw() {

  background(255);

  if(startColor > width + 255 || startColor < 255) {

    speedAnimate *= -1;

  }

  startColor += speedAnimate;

  let gap = 1;

  let numRectangles = 20;

  let rectWidth = width / numRectangles;

  for (let x = 0; x < width; x += gap + rectWidth) {

    let green = startColor - x;

    let red = 255;

    fill(red, green, 0);

    rect(x, 0, rectWidth, height)

  }

}