Bom dia, assim ficou minha bandeira.
<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(300,25);
pincel.lineTo(50,200);
pincel.lineTo(300,375);
pincel.lineTo(550,200);
pincel.lineTo(300,25);
pincel.fill();
pincel.fillStyle = 'blue';
pincel.beginPath();
pincel.arc(300, 200, 125, 0, 2*3.14);
pincel.fill();
pincel.fillStyle = 'white';
pincel.fillRect(175,190,250,20);
</script>