só o básico...
<canvas width="800" height="600"></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'gray';
pincel.fillRect(0,0,800,600);
function circulo(x,y,raio, cor){
pincel.fillStyle = cor;
pincel.beginPath();
pincel.arc(x, y, raio, 0, 2*3.14);
pincel.fill();
};
circulo(400,300,50, 'red');
circulo(300,300,50, 'blue');
circulo(400,200,50, 'yellow');
circulo(500,300,50, 'pink');
circulo(400,400,50, 'purple');
</script>