When the paint had dried, Ludo had turned quite blue in the face, which is just wrong. So, I had to start over, this time with Davy's Grey instead of Payne's Grey as the base colour. The mixture of grey and white, predictably, was entirely unable to cover the darker grey underneath, so I spent yet another week waiting the the initial coat to dry sufficiently.
The mixed up paints survived that time in perfectly workable condition, so I could continue with a second coat today. Davy's Grey is the perfect neutral grey, which means it looks absolutely ghastly on anything organic, so I added just a bit of Burnt Sienna to the mixture, and for the second coat, blended that onto the skin. This still left the lighter part looking too dead, and too light when compared to the original, so I dumped in some Raw Umber to make it darker, without adding more red.
All that fiddling with paint then left me with this:
Not too bad, but I'll be doing some further touch-up, and adding a bit of red to his cheeks, in a few days.
If anyone here still thinks human skin is a problem to paint, wait till you try non-human stuff