Cambiando de color

<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); 
    colores= ["blue","red","green"]

    function exibirAlerta(evento){
     var x = evento.pageX - pantalla.offsetLeft;
     var y = evento.pageY - pantalla.offsetTop;
     console.log(evento);
     alert(x + ","+y)

    }
    pincel.fillStyle = colores[0];

    function dibujarCirculo(evento){
     var x = evento.pageX - pantalla.offsetLeft;
     var y = evento.pageY - pantalla.offsetTop;
     pincel.beginPath();
     pincel.arc(x,y,10,0,2*3.14);     pincel.fill();
    }

    pantalla.onclick = dibujarCirculo;
    i=1
    function alterarColor() {
        pincel.fillStyle=colores[i]
        i++
        if(i==3){i=0}
        return false;
    }

    pantalla.oncontextmenu = alterarColor;

</script>