For the eyes, I used the construction technique described in this cool tutorial, here on 3DTotal:
http://www.3dtotal.com/index_tutorial_detailed.php?id=948. I just used a bit of displacement for the iris. I made the grayscale displacement by painting over my eye texture, defining bumps and holes here and there.

Texturing

No photo projection was used to paint the diffuse texture of the face. Everything was made from scratch.
Here's how I did it:

I turned the color of the material to something very different from human skin, like green, which helped me to see if I'd covered the whole surface during the painting process. Then I painted a base color similar to human skin (Fig.15).


And now, for the fun part! Using stamps and stencils, I started making some color variations on the same layer, using the Burn and Dodge. When doing this it's important to always keep in mind that you should work with stamps for the whole process, to keep having some noise and variations. Areas such as upper head, cheek bones and chin received more "burning” than the others, as they are more exposed to sunlight (Fig.16).



This stage took a long (long, long) time. It was all about fine tuning and testing. If I went a bit too far in the burning or dodging process, I used the airbrush and my base color to paint over the areas I wanted to attenuate.

Once this layer seemed OK, I created some other layers to add color variations to the skin, veins, spots, aging marks, freckles etc. I kept in mind that human skin is not only composed of red-pink-yellow colors, but you can also find areas with touches of purple, blue, green or gray tones (around the eyes, on the cheeks for males with shaved beard, etc). Once again, it was crucial to have as much reference material as possible at this stage too.

I used a different layer to paint the mouth color, which allowed me to tune it separately from the rest in Photoshop when it came to testing the look of the map during renders.

Here are some screenshots of the final painting into the mudbox viewport (Fig.17).


Here are the same pictures with flat lighting, so you can see the details better (Fig.18).