Since the goal was to create an illustration, I didn't need to really texture my model as if it was intended to end up in a cinematic game or movie. I simply wanted to create a concept and quickly visualise the model as a final product.
So to do this, I just used the automatic AUV tile inside ZBrush. Like I said, there was no need to bother with clean UVs and unwrapping because I wasn't intending on painting on the flat UV template, but on the actual 3D sculpt instead, using polypaint (you can find out more information about the tool on ZBrush: http://www.zbrush.info/).
Since I planned the look of the character in my initial sketches, I already knew what I had to do at this stage. I had to split my basic texture into two layers; the first one was obviously for the tattoo, and the second was for the skin tone colour. For the tattoo, I extensively used the Lazy tool, which helps you to control your brush strokes more precisely. For the skin, I used a painting technique explained by Scott Spencer, which basically consists of painting the skin colour in layers. Depending on the area, you paint in blue, red or yellow, and then finally cover everything with a thin tonal layer of brown/orange. This is a really effective technique, I must say!
Once my two maps were ready, I thought about the different passes I would need. I came up with these main passes: an Occlusion, Specular, Reflection, ZDepth and Mask pass (Fig.03). These passes were achieved simply by assigning a specific MatCap to the model which mimicked the desired effect. I saved each render separately by exporting the doc.
For information, there is a great MatCap repository thread on ZBrush Central, but you can, of course, create your own MatCap. If you're interested in this, simply take a look on ZBrush Central - just look at the ZBrush Info page, there's a great tutorial there that clearly explains the process of how to create your own MatCap.
The ZDepth pass is really easy to get: go to the alpha palette, click on grab doc, and then save the document. For the Mask pass, I assigned a colour to each SubTool and then selected the flat material. This render was useful to be able to later select the different object easily.
With all the texturing covered, let's now go on to discuss the compositing work, which was all done in Photoshop.
During this phase, a lot of experimentation was necessary. There were few common things though, such as the specular and occlusion render, which were going to be set respectively to Screen and Multiply modes. As for the other passes, they were, most of the time, set to Overlay or Soft Light modes. Keep in mind however that experimentation with the other modes is the best way to achieve an interesting look.
See Fig.04 to see how I managed my layers for this project. Once all of my basic passes were composited, I began adding some photos of leather, metal and dirt on top of it. Again, test the different blending modes to suit your personal aims and objectives.
The purpose of this step was to apply texture information and to add a touch of realism to the image. I also hand-painted some elements like the drool going on inside his mouth, as well as some highlights and shadows here and there. Finally, I used a filter, such as the Lighting Effect one, and a photo filter to give the overall image a uniform feel. Radial Blur was also used to add some movement and depth to the image. And here is the final result (Fig.05).