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