<canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'red';
pincel.fillRect(0,0,600,400);
pincel.fillStyle = 'black';
pincel.beginPath();
pincel.moveTo(50,50);
pincel.lineTo(50,400);
pincel.fill();
</script>