let xspeed = 5;

let color;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  color = map (x, 25, 375, 255, 0);

​

  fill (color);

  ellipse(x, height/2, 50, 50);

  x += xspeed;

  if (x <= 25 || x >= 375) {

    xspeed = - xspeed;

  }

}