Inténtalo tú mismo - HTML Games component_color

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
<style>
canvas {
    border:1px solid #d3d3d3;
    background-color: #f1f1f1;
}
</style>
</head>
<body onload="startGame()">
<script>

var myGamePiece;

function startGame() {
    myGamePiece = new component(30, 30, "rgba(0, 0, 255, 0.5)", 10, 120);
    myGameArea.start();
}

function component(width, height, color, x, y) {
    this.width = width;
    this.height = height;
    this.x = x;
    this.y = y;    
    this.update = function(){
        ctx = myGameArea.context;
        ctx.fillStyle = color;
        ctx.fillRect(this.x, this.y, this.width, this.height);
    }
}

function updateGameArea() {
    myGameArea.clear();
    myGamePiece.x += 1;
    myGamePiece.update();
}
</script>
<p>Look! A blue transparent little square!</p>
</body>
</html>