Check out http://kuku-kube.com/

kuku_kube_helper.js

/*jslint browser: true, indent: 2 */
(function () {
  'use strict';
  var rgb, style, idx = 0, store = {}, tiles = document.getElementById('box').childNodes;
  for (idx; idx < tiles.length; idx += 1) {
    rgb = tiles[idx].style.backgroundColor.split("(")[1].split(")")[0].split(",");
    store[Math.sqrt(Math.pow(rgb[0], 2), Math.pow(rgb[2], 2), Math.pow(rgb[2], 2))] = tiles[idx];
  }
  style = Object.keys(store).map(function (key) { return [key, store[key]]; }).
    sort(function (a, b) { return parseInt(a[0], 10) < parseInt(b[0], 10) ? 1 : -1; })[0][1].style;
  style.border = '2px solid white';
  style.outline = '2px solid black';
}());

minified for KuKu Kube Bookmarklet:

javascript: !function(){"use strict";var t,e,o=0,n={},r=document.getElementById("box").childNodes;for(o;o<r.length;o+=1)t=r[o].style.backgroundColor.split("(")[1].split(")")[0].split(","),n[Math.sqrt(Math.pow(t[0],2),Math.pow(t[2],2),Math.pow(t[2],2))]=r[o];e=Object.keys(n).map(function(t){return[t,n[t]]}).sort(function(t,e){return parseInt(t[0],10)<parseInt(e[0],10)?1:-1})[0][1].style,e.border="2px solid white",e.outline="2px solid black"}();