I added in my blog a rather long step-to-step tutorial about how to add “Decals” to a mesh.
In my lingo, Decals are auxiliary transparent meshes used to add local texture detail to a different base mesh. For example, in the render below the pink spots on the cheeks of this toon girl are Decals, not part of the face mesh.
Link to the step-by-step tutorial:
This tutorial is intended for Blender Internal. Under Cycles the workflow would be quite different. I haven’t explored that yet, unfortunately.