There are many different techniques and ways of creating a selective color images in Photoshop, let me show you the one where we use the black & white adjustment layer.
A bit longer Photoshop tutorial, but still very suitable for beginners, where you will learn how to carve a random text into human skin from scratch and make it look realistic.
If you were ever wondering how the heck do they create those continuous patterns, then wonder no more and check this neat Photoshop tutorial that might teach you a new designing trick or two.