Easy way of doing sick flesh / unhealthy skin

Zenithal highlight priming (optional)

To make it easier for yourself make a zenithal prime, this is optional but simplifies a lot.

1. Prime with black primer
2. Zenithal prime from the top with white primer from the spray can or airbrush
3. Wait 6-24 hours for primer to dry and settle

Painting skin

1. Basecoat shadows (all the areas that have black primer) with Topaz Skin
2. Basecoat all highlights (all light gray and white primed areas) with Barbarian Flesh

Midtones (darker gray areas) can be basecoated with either Topaz Skin or Barbarian Flesh it is up to you.

1. Now you can either use Dorado Skin for the next step or you can play around a little and create a mix of Barbarian Flesh , Skeleton Bone and Necrotic Flesh to get something close to Dorado Skin in lightness but with more sickly tones.
2. With that mix (or pure Dorado Skin ) highlight part of the areas that were painted with Barbarian Flesh . Try to do volumetric highlights (not edge highlights).
3. Dilute one part Rigor Mortis with two parts of Speedpaint Medium , then wash the whole skin with that mix, but don't let it pool too much. Wick excess paint with cleaned brush. In other words we are doing something like heavy glaze/light wash.

Interesting property of Rigor Mortis is that it doesn't darken like other speedpaints, rather lightly brightens darker undertones and unifies all the skin. This creates interesting variety in the whole skin, that can be read as being unhealthy.

This skin can be treated as a base and refined further, for example to get more stylised look. Here I've added some dark bluish glaze / paint in the shadows. I also refined midtones / highlights with Barbarian Flesh , Necrotic Flesh and Skeleton Bone