<canvas width="900" height="600"></canvas>
<script >
var pantalla = document.querySelector("canvas");
var pincel = pantalla.getContext("2d");
pincel.fillStyle = "blue"
pincel.fillRect(0,200,900,600);
pincel.fillStyle = "yellow"
pincel.fillRect(0,0,900,200);
pincel.fillStyle = "red"
pincel.fillRect(0,400,900,600);
</script>