Ejercicio: Cambiando de color

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

<script>

    let color = ["blue","green","red"]
    let modificarColor = 0;

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

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

    pantalla.onclick = dibujarCirculo;

    function alterarColor() {
        modificarColor ++;
           if (modificarColor >= color.length) {
               modificarColor = 0;
           }
           alert("Funcionó " + color[modificarColor])
           return false;
    }

    pantalla.oncontextmenu = alterarColor;

</script>