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