createCanvas(400, 300);

  noStroke();

  textSize(20);

}

​

function draw() {

  background(220);

  text(mouseY, 20, 20);

  if (mouseY < 100) {

    // red light

    fill('red');

    circle(200, 50, 100);

  } else if (mouseY < 200) {

    // yellow light

    fill('yellow');

    circle(200, 150, 100);

  } else {

    // green light

    fill('green');

    circle(200, 250, 100);

  }

}