ok, let me get this right.
These are painted directly on to those models via the UV map, using a tablet. I guess your using photoshop and tools like dodge and burn and so on.
(like my bro does

). And I'm guessing you have no colour limit as the colours go over 256. So Verbit, look up texture art. Making wood, metal and so on, and you'll have the basics of what your looking for.
Most of the detail looks waisted on these, as they look so pixelated, because of the redering size of the model.
But well that was for you to decide.
Again these are just awesome and I look forward to more.