Comparto Mi Codigo para cambiar colores

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

<script>

    var pantalla = document.querySelector("canvas");
    var pincel = pantalla.getContext("2d");
    var colores = ["blue","red","green"]
    var color = "Blue"    
    pincel.fillStyle = "grey";
    pincel.fillRect(0,0,600,400);

    function alterarColor() {
        if (color == colores[0]) {
            color = colores[1]
        }
        else if (color == colores[1]) {
            color = colores[2]
        }
        else  {
            color = colores[0]
        }

         return false;
    } 

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

    pantalla.onclick = dibujarCirculo;




    pantalla.oncontextmenu = alterarColor;


</script>