Taking an existing painting or illustration and trying different color variations can be a very interesting project and is a good way to hone your computer skills. Just let your imagination run wild and see what happens.
Above is one of 36 black and white illustrations that I did for Ben Leibson's book, "Drifting Sands". It was created on a Mac 4 computer. Many interesting variations can be created of an image like this easily in photoshop, it's interesting as well as a great learning experience.
Here's another black and white version, I could have gone on and done dozens of these, the computer makes this type of experimentation comparatively easy.