A Random(r, g ,b) color generator

I've just starting to learn JavaScript, So I know only a few things about it. I have searched this question and I found the right code for it that works fine with hexadecimal color format. But before that i was trying sth on my own, Can you help me to turn it to an RGB color generator. Every time I add this code to my page it won't work.

      document.getElementById("myButton").onclick = function() {
        var r =Math.random();
        r = 255*r;
        r = Math.floor(r);

        var g =Math.random();
        g = 255*g;
        g = Math.floor(g);

        var b =Math.random();
        b = 255*b;
        b = Math.floor(b);

        var color= "rgb(" + r +"," + g +"," +b ")";

        document.getElementById("myDiv").style.background: color;
     }