This was done for the Hardcore Modeling Challenge a few months back. It was a concept art that I did before I sculpted the real head. I liked how the concept art turned out so I decided to refine just a little bit but I wanted to still keep some elements rough and painterly.
I only manage to complete Leonardo for the challenge so this became a another personal project to finish. Currently I am still exploring some more concepts for him.
I roughly sculpted a basic basemesh in Zbrush and painted over it in Photoshop.