<meta charset="UTF-8">
<body>
<canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext('2d');
pincel.fillStyle = 'green';
pincel.fillRect(0, 0, 600, 400);
pincel.fillStyle = 'yellow';
pincel.beginPath();
pincel.moveTo(300, 10);
pincel.lineTo(590, 200);
pincel.lineTo(300, 390);
pincel.lineTo(10, 200);
pincel.fill();
pincel.fillStyle = 'darkblue';
pincel.beginPath();
pincel.arc(300, 200, 115, 0, 2 * 3.14);
pincel.fill();
</script>
</body>
Resultado: