<canvasid="testcanvas"width="500"height="500"></canvas>
<script>
var c =document.getElementById('testcanvas');
var p = c.getContext("2d");
p.fillStyle ="#FF0000";
p.fillRect(0, 0, 300, 300);
p.fillStyle ="rgba(0,0,255,0.5)";
p.fillRect(200,200,500,500);
</script>