var black;

​

function setup() {

  createCanvas(200, 200);

  background(0);

  button = createButton('click me');

  button.position(19, 19);

  button.mousePressed(changeBG);

  button = createButton('reset');

  button.position(19, 49);

  button.mousePressed( function() {

    background(0);

  });

}

​

function changeBG() {

  var c = color(random(255), random(255), random(255));

  background(c);

}

​