Use Photoshop gradient color maps as a base to color your creatures.
Angelina Diez, character artist and 3D modeler from Spain, is working on a series of tutorials dedicated to hand-painted textures. The artists explains how to do this kind textures with Photoshop gradient color maps as a base. You will also need 3D Coat to follow her instructions.
Angelina so far has uploaded three parts and more videos are coming, so make sure to follow her on YouTube. Here are the second and the third parts: