Cambiando de color

<canvas width="600" height="400"></canvas>
<script type="text/javascript">
var pantalla = document.querySelector("canvas");
var pincel   = pantalla.getContext('2d');

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

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

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

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

        pincel.fillStyle = colores[indexColor];
        pincel.beginPath();
        pincel.arc(x,y,10,0,2*3.1416);
        pincel.fill();
        console.log(x + "," + y);


    }

    pantalla.onclick = dibujarCirculo;

    function alterarColor(){
        indexColor++;
        if(indexColor >= colores.length ){
         indexColor = 0;
        }
        console.log("Funciono");
        console.log(indexColor);
        //alert("Funciono");
        return false;
    }

    pantalla.oncontextmenu = alterarColor;


</script>