Click Change Background. Using p5.js library.
var bgcolor;
function setup () {
createCanvas(1024, 576);
bgcolor = color(200);
createButton("Change Background");
}
function mousePressed () {
bgcolor = color(random(0, 170), random(0, 170), random(0, 170));
}
function draw () {
background(bgcolor);
fill(255, 0, 0);
rect(200, 200, 200, 200);
fill(255);
textSize(60);
text('Click on background of canvas.', 100, 100);
}