SOURCE

let x=10;
let y=10;
let d=10;
let a=10;
let b=10;
function setup(){
    createCanvas(450,500);
    noStroke();
    colorMode(HSB,5,26,200)
}
function draw(){
    //background(120,10);
    x+=a;
    y+=b;
     fill(random(1),random(35),random(150));
    if(x>width+d/2||(x<d/2)){
        a=-a;
    }
    if(y>height-d/2||(y<d/2)){
        b=-b;
    }
    ellipse(x,y,d,d);
   
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>