Mitch ... not sure if you're going to be reading this but before getting into air brushing which started because of my interest in special effects make up (eg scars, wounds, bullet wounds, burn effects, zombie effects etc) and this was supposed to be an adjunct to that hobby interest but has quickly taken over it ... I was doing a lot of work on skin textures.
In SPFX blending a prosthetic piece eg a bullet wound after "sticking" it onto the skin is the next basic skill after actually making the prosthetic. Mind you, it's not easy and I was in the process of learning that still when my wife got some cake decorating airbrush stuff and because you use airbrushing in SPFX as well (which lead me to want to get a handle on it - but now I'm friggin addicted....) I ended up getting more stuff for my purposes and then one thing lead to another - anyway I digress......
The first thing you learn is that the skin is in fact transparent and made up of layers of colour. You start with matching up as best you can the lightest colour of the area and then you add "stippling" of colours usually of RED, BLUE and GREEN (all of which are very pale colourings. After that, you apply brown for skin blemishes (dots) and then black stipples for possibly hair and stubble, follicles etc
To get an idea of the colours google "Skin Illustrator Skin Tones" pallet. This is the one we buy and use to get the appliance (as they're also called in the SPFX industry) to start the stippling effect with. You start OFTEN but not always. with red and sometimes followed by green (but this is for a olive complexion) and the colours chosen do depend on the skin you are blending the appliance on to. Have a look at the colours on the skin illustrator palette and hopefully this may either send you in a new direction OR just send you on a wild goose chase.
DISCLAIMER: This is general advice and maaate .... if you follow these suggestions then ummm errr all time you spend on this pursuit is not to be billed to me
Good luck Jim and as usual if you or any of your team get caught ...................................