Cambiar color del circulo

<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);

  let colores = ["blue", "red", "green"];
  let index = 0;

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

  pantalla.onclick = dibujarCirculo;

  function alterarColor() {
    index++;
    if (index >= 3) {
      index = 0;
    }
    return false;
  }

  pantalla.oncontextmenu = alterarColor;
</script>