<canvas wiadth="600" height="400"></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext('2d');
pincel.fillStyle = 'lightgrey';
pincel.fillRect(0, 0, 600, 400);
pincel.fillStyle = 'green';
pincel.fillRect(0, 0, 200, 400);
pincel.fillStyle = 'red';
pincel.fillRect(400, 0, 200, 400);
</script>