<canvas id="canvas" width="1024" height="600"></canvas>
<script type="text/JavaScript">
var ctx = document.getElementById('canvas').getContext('2d');
var r = Math.floor((Math.random() * 256));
var g = Math.floor((Math.random() * 256));
var b = Math.floor((Math.random() * 256));
ctx.fillStyle = 'rgb(' + r + ',' + g + ', ' + b + ')';
ctx.fillRect(10, 10, 90, 120);
</script>