Cambio de color

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

<script>

    var pantalla = document.querySelector("canvas");
    var pincel = pantalla.getContext("2d");    
    pincel.fillStyle = "Dimgrey";
    pincel.fillRect(0,0,600,400);

    var colors = ["blue","Red", "LawnGreen",]
    var posicioActual = 0


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

    pantalla.onclick = dibujarCirculo;

    function alterarColor() {
        posicioActual++
      if (posicioActual >= colors.length) {
          posicioActual = 0 
    }  

        return false; 
    }






    pantalla.oncontextmenu = alterarColor;

</script>