Hello Gareth,
nothing more I can add. Both the Bill Horan masterclass book and Mike Blanks' "Black and White" have been my biggest influence, and their work (certainly together with Adrians') caused me to try out enamels for faces.
Keith, thanks for putting that article! In fact, the faces on that dio are the first ones I ever did in enamels. I felt it to be a huge improvement over my earlier work in oils.
The colours in it are mostly "borowed" from Mike. Nowadays, I mix in some Leather 62 if I want to get a warmer or more tanned complexion though.
If you have any question, just shoot!
Cheers!
Marijn