This is a port of the code from here to JavaScript (modified slightly for performance reasons). It depends on the <canvas> tag, so you'll need a web browser like SeaMonkey 1.0, Firefox 1.5, Camino 1.0, Safari, or newer (I only tested with SeaMonkey though). Your browser is probably going to hang for a couple of seconds when you click "Begin".
Colorize (SLOW)