This solution is totally different, building up the color emoji using a cool font table approach. You should read it; it's worth the time. :)
Actually it is a pretty dirty hack -- it seems ok because the flat design is currently popular. But quite likely gradients and shadows will come back in a few years and then few single-color layers won't be enough.
You could build gradients with this, but it wouldn't look nice zoomed in. And, importantly, Apple's current gradient-filled font wouldn't work!