Oh wow, completely forgot about this thread. Sorry about that. How is it going with the painting? Sorry I didn't give more and better advice.
For regular skin tones, you should check this site -
http://www.theeffectslab.com/skinpainting.htm - in case it's not too late and you've sold the bust in the meantimes, since this is a 2-year-old thread and I don't know the current status.
What is needed is layering. Subtle layering with veining and spots and break-up of perfect uniform color palette. Since he's burnt, scarred, having healed up, and having lived within a helmet for the better part of 40 years or something, you should do a pale damaged skin with lots of veining showing and dark blended bags under his eyes and very light red and brown and white for the scar tissue. Basically... look at pictures of old nasty, gnarly scars to see if you can distinguish colors and how to paint it to make things convincing.
So... how's the bust coming along or did you already paint it?