<canvas width="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);
</script>
