I know what look you are after. I havn't tried this formula myself, but this is what I would do and see how it comes out:
Paint the whole thing flat black.
Heavily drybrush a very dark maroon.
Repeat with a slightly different shade, maybe a little lighter, maybe a little more brownish.
Lightly drybrush dark grey.
very lightly drybush white or light grey.
Also, that expanding foam insulation in an aerosol is very pourus when cut. It may be benificial to use this, then cut it, perhaps in isolated areas. Experiment outdoors and off layout with this stuff, it's nasty.
The volcanic isles are very fertile, and greenery should be quite abundand if it has been very long since hte last eruption.
Maybe you can find some nice images here:
http://images.google.com/images?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=hawaii&sa=N&tab=wi
or here:
http://images.google.com/images?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=philippines&sa=N&tab=wi
I don't have a very good picture right now, but the foam reminded me of the lava in the philippines. This image shows it a litle bit, wrong colors and I filled in a lot of the pores with paint, as they were not the desired effect.
