Conclusão do código - Alteração usando array's

<meta charset="UTF-8">

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

<script>

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

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

    var cores = [ "blue", "red", "green" ];
    var cor_atual = 0;

    function onClick_left ( event ) {
        var x = event.pageX - tela.offsetLeft;
        var y = event.pageY - tela.offsetTop;
        pincel.fillStyle = cores[cor_atual];
        pincel.beginPath ( );
        pincel.arc ( x, y, 10, 0, 2 * 3.14 );
        pincel.fill ( );
    }
    tela.onclick = onClick_left;

    function onClick_right ( event ) {
        if ( cor_atual >= cores.length - 1 ) {
            cor_atual = 0;
        } else {
            cor_atual++;
        }
        //console.log ( cor_atual );
        return false;
    }
    tela.oncontextmenu = onClick_right;

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