1. Take any graphic you want; the simpler, the better.
2. Go to the Layers tab, right click, and choose Duplicate Layer
(You'll get a message asking what you want to name the layer and where you want it. Just press OK to go with the default.)
3. Go to the Image Section in the top area, select Adjustments, then Desaturate.
4. Now, here's what my graphic looks like so far:
5. I'm going to make the butterfly stand out, so I'm going to select a small Eraser brush, this one being Oval 10 px...take your time, and even zoom in a little bit to see that you're only revealing the color where you want to.
Voila! Finished Product: