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); 
    var color = ["blue","red","green"];
    var i =0; 

    function dibujarCirculo(evento){
        var x = evento.pageX - pantalla.offsetLeft;
        var y = evento.pageY - pantalla.offsetTop;    
        pincel.fillStyle = color[i];
        pincel.beginPath();
        pincel.arc(x,y,10,0,2*3.14);
        pincel.fill();
        console.log(x + "," + y);        
    }


    pantalla.onclick = dibujarCirculo;

    function alterarColor() {
        i++
        if(i==color.length){
            i=0;
        }

        alert("ahora su color cambio a: "+ color[i])  
        return false;

    }

    pantalla.oncontextmenu = alterarColor;

</script>