Ejercicio color en circulos

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

<script>
    var pantalla = document.querySelector("canvas");
    var pincel = pantalla.getContext("2d");

    pincel.fillStyle = "grey";
    pincel.fillRect(0,0,600,400);
    var color= ["blue","red","green"];
    var indice = 0;


    function exibirAlerta(evento){
        var x = evento.pageX - pantalla.offsetLeft;
        var y = evento.pageY - pantalla.offsetTop;

        console.log(evento)
        alert(x + "," + y);
    }


    function dibujarCirculo(evento){
        var x = evento.pageX - pantalla.offsetLeft;
        var y = evento.pageY - pantalla.offsetTop;


        pincel.fillStyle=color[indice];
        pincel.beginPath();
        pincel.arc(x,y,10,0,2*3.14);
        pincel.fill();
        //console.log(evento)
        console.log(x + "," + y)

    }

    function alterarColor(){

        indice++;

        alert(indice);

        if (indice >= color.length){
            indice = 0;
        }
        return false;
    }

    pantalla.onclick = dibujarCirculo;
    pantalla.oncontextmenu = alterarColor;

</script>