Fiz a minha bandeira sem precisar colocar duas partes Amarelas:
<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 = "green";
pincel.fillRect(0, 0, 200, 400);
pincel.fillStyle = "yellow";
pincel.beginPath();
pincel.moveTo(300, 50);
pincel.lineTo(50,200);
pincel.lineTo(300,350);
pincel.lineTo(550,200);
pincel.fill()
pincel.fillStyle = "darkblue";
pincel.beginPath();
pincel.arc(300, 200, 100, 0, 2 *3.14);
pincel.fill()
</script>
