<!DOCTYPE html>
<html>
<body>
<canvas id ="output" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById('output');
var ctx = canvas.getContext('2d');

var colors = ['red', 'green', 'blue'];
for (var i = 2; i >= 0; i--) {
    ctx.fillStyle = colors[i];
    ctx.fillRect(i * 20, i * 20, 40, 40);
}
</script>
</body>
</html>