Edit in JSFiddle

// helper function for displaing results
var output = function(operator, result) {
    document.getElementById(operator).innerHTML = result;
}

// variables
var color = {r: 186, g: 218, b: 85};

// RGB to HEX
var rgb2hex = function(r, g, b) {
    return '#' + ((1 << 24) + (r << 16) + (g << 8) + b).toString(16).substr(1);
}

output('color', rgb2hex(color.r, color.g, color.b));
rgb(186, 218, 85);<br/>
Result: 
<p>rgb(186, 218, 85) = <span id="color"></span></p>
 
span {
    color: #f00;
}