Thanks! The skin tones were half by accident - I primed them white then gave them a wash with thinned down Agrax Earthshade, mainly so I could see the details better. I found that the thinned Agrax already looked a bit like really unhealthy skin, so I added thinned down Reikland Flesh, especially around the eyes, mouth, and anywhere the skin was broken or flesh was showing. After that I blocked in everything else with colours that could fit a range of periods, and picked out some details like the eyes and teeth.
I’m just hoping I can replicate the effect on the next batch!