add recolor to make Pop Art

I want to make a pop art photo with wand, but not found any function to manipulate pixels.

The example is here: https://github.com/beartung/pop_art_wand

looks like this:

![test_1886982_dec](https://f.cloud.github.com/assets/129518/457529/083bf658-b3a3-11e2-8fbb-a307d56d40bb.jpg)