Bolinhas coloridas

<canvas width="600" height="400"></canvas>

<script>
    var tela = document.querySelector('canvas');
    var pincel = tela.getContext('2d');
    var cores = ['blue', 'red', 'green']
    var poscor = 0

    pincel.fillStyle = 'grey';
    pincel.fillRect(0,0,600,400);

    function desenhaCirculo(evento) {

        var x = evento.pageX - tela.offsetLeft;
        var y = evento.pageY - tela.offsetTop;

        pincel.fillStyle = cores[poscor]
        pincel.beginPath()
        pincel.arc(x, y, 10, 0, 2 * 3.14);
        pincel.fill();
        console.log(x + ',' + y)

    }

    tela.onclick = desenhaCirculo;

    function mudacor() {


        poscor = poscor + 1

        if (poscor >= cores.length) {

         poscor = 0
        }    

        return false;
    }

    tela.oncontextmenu = mudacor;

</script>