TROCANDO A COR DA BOLINHA

<meta charset="utf-8">

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

<script type="text/javascript">

    var tela = document.querySelector('canvas');
    var pincel = tela.getContext('2d');

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

    var cores = ['blue','green','red'];
    var indiceCor = 0;

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

        pincel.fillStyle = cores[indiceCor];
        pincel.beginPath();
        pincel.arc(x,y,10,0, 2*Math.PI);
        pincel.fill();
    }

    tela.onclick = exibeBolinha;

    function mudarCor(){
        indiceCor++;
        if(indiceCor >= cores.length){
            indiceCor = 0;
        }
        return false;
    }

    tela.oncontextmenu = mudarCor;

</script>