On a number of projects over the past year or two, I've been working on my mix for unnatural skin. What exactly do I mean by unnatural skin? Well, something that looks human, but not quite right. Maybe it's an evil wizard, a warrior warped by the powers of chaos, a vampire, etc. You could take the same ideas and apply them to orcs, aliens, etc, but my focus is something that looks human-ish, but not quite right.
So here is a look at a couple of my projects and the colors I used. The most recent one (and the one I think has finally achieved the look I want) is the dwarf pirate. Here's a close look at his face and also his hand.
My normal skin mix (for regular human skin) is Reaper's Chestnut Brown, Rosy Shadow, Fair Skin, and Fair Highlight. For this guy, I tweaked that a bit. I used a mix of Rosy Shadow and Burgundy Wine for the shadows. More Burgundy wine for the darkest shadows and then maybe 10-20% Burgundy Wine for the lighter ones. Into that mix (90/10 Rosy Shadow and Burgundy Wine), I started to add my midtone, 50/50 mix of Fair Skin and Vampiric Shadow. To highlight I then used Vampiric Skin and Vampiric Highlight. You can see what some of those colors look like in the image below. Burgundy Wine is on the far right and Vampiric Shadow is in the front left.
This mix gives you some cooler shadows (purple tones instead of healthy reddish browns) and creates a paler skin by mixing the fair skin with vampiric skin.
Now, on top of that I applied a number of glazes. Normally I use red, purple, and blue ink (red for the cheeks and nose, purple for the darkest shadows and under the eyes, and blue for the stubble). In this case I shifted those colors. So, instead of red, I used two parts red and one part purple. Instead of purple, I used 1 part purple and 1 part blue. This reddish purple went in the cheeks, under the nose and on the knuckles, the bluish purple went in the shadows and under the eyes, and the pure blue ink went in the veins on the back of the hand.
By taking an approach similar to what I use for normal skin, the end result reads as flesh. But, by shifting the colors, I create a different effect. Compare the dwarf pirate to a normal head I painted a while ago. The technique and approach are the same, it's just the colors that are different.
Let's take a look at another mix I used for unnatural skin. This is a dark elf assassin I painted last year. In this instance I used Dusky Skin, Dusky Skin Highlight, and Vampiric Shadow to create the skin. I applied similar glazes (again shifting towards purple and blue). The glazes are subtle, but you can see them under the eyes and in the cheeks. I think that approach worked well for a figure that was mostly in shades of grey, but I prefer the effect achieved by mixing in some regular skin tones and going a bit further with the glazes.
Okay, this one is not unnatural skin, but I wanted to talk a little bit about it anyway. This is a bust I started a while ago. As I said, my normal mix is Chestnut Brown and Rosy Shadow. In this case I also added a little bit of Burgundy Wine to both of those shades. It wasn't enough to make him look unnatural, but it took a little bit of the health and vitality from the shadows. I guess the point I'm trying to make is that there is a whole range you can work with. You can tweak the mixes a little bit or a lot depending on the effect you want. Maybe you tweak just the shadows by adding blues or purples. Or perhaps you adjust only the lighter colors and make them more pale. Experiment on your own and decide what is right for your figure.