mi respuesta

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

<script>

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

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

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

    pantalla.onclick = dibujarCirculo;

    function alterarColor() {


        if (iColor>=2){
            iColor = 0;
        } else {
            iColor= iColor + 1;
        }

        alert("Nuevo color: "+ colores[iColor]);

        return false;
    }

    pantalla.oncontextmenu = alterarColor;

</script>