let xSpeed = 3;

let startcolor = 255; 

let colorspeed = 3;

​

function setup() {

  createCanvas(400, 400); 

}

​

function draw() {

  background(220); 

  x = x + xSpeed;

  if (x > width || x < 0) {

    xSpeed *= -1;

    colorspeed *= -1; 

  }

​

  startcolor += colorspeed;

  startcolor = constrain(startcolor, 0, 255);

  fill(startcolor);

  ellipse(x, height / 2, 20, 20);

}

​