Colores

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

<script>

        var pantalla = document.querySelector("canvas");
        var pincel = pantalla.getContext("2d");
        var colores = ["blue", "red", "green", "yellow"];
        var indiceColor = 0

        pincel.fillStyle = "grey" ; 
        pincel.fillRect(0,0,600,400);

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

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

    }

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


        pincel.fillStyle = colores[indiceColor];
        pincel.beginPath();
        pincel.arc(x,y,10,0,2*3.14);
        pincel.fill();
        console.log(x + "y" + y);

    }

    function cambiarColor(){
        indiceColor++;
        while(indiceColor >=colores.length){
            indiceColor = 0;
        }
        return false;
    }

   // pantalla.onclick = exibirAlerta;

       pantalla.onclick = dibujarCirculo;
       pantalla.oncontextmenu = cambiarColor;

</script>