Hi Erik,
Apologies for the delayed response to this! Mike did pester me about this for a while, and I then promptly forgot.
However, the main thing to do is not worry about all the fancy stuff and get the basics down properly - if you're wanting to paint up the figures like the standard ones.
So, for the majority of the figures whose skin is on the brown end of the spectrum, I tend to start with a base colour of:
GW Baneblade brown.
It's a staple colour and it forms the basis for all manner of my stuff. It's also a good midtone colour that you can shade easily and highlight similarly.
Fubarnii are pretty much this as the base, then a brown wash (Agrax Earthshade will do the job perfectly well). I tend to dilute it just a little bit with some water, but all i can advise you to do in this regards is test it out. As long as you don't apply the stuff too thickly, it should be fine.
It's then a task of using the original brown over this, and gradually work up the highlights with diluted paint - don't use white if possible. Use something like a Rakarth Flesh (another GW one). I'd then possibly glaze this all over with a mix of brown and maybe a little purple in the mix to give the skin a bit more life, but that's pretty much it.
(I've over simplified, but that's the basis of it!).
With the casaanii, the skin was pretty much exactly the same, but the base coat was baneblade brown, plus a little bit of more 'red' brown in there too - something like Mournfang brown does the job. I'd then wash this with brown again, and then the same sort of thing highlighting up the levels. I would then probably go over this with a glaze of something like Reikland flesh to give things a little more of a more 'tanned' effect than the Fubarniis.
I can certainly elaborate more if you want, but all i can say is paint something up and show it off here and we can see what happens!
Cheers