SOURCE

let x = 51;
let y = 51;
let d = 50;
let speedx = 3;
let speedy = 4;
let c=0;
function setup(){
    createCanvas(500, 500);
    noStroke();
    colorMode(HSB,360,100,100);
}
function draw() {
    //background(120);
    c++;
    if(c>360){
        c=0;
    }
    x += speedx;
    y += speedy;
    if((x > width-d/2) || (x < d/2)){
        speedx =  - speedx;
    }   

    if((y > height-d/2) || (y < d/2)){
        speedy =  - speedy;
    }
    fill(c,100,100);
    ellipse(x, y, d, d);
}

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