It's been a while but much of it was achieved with washes over a white or grey basecoat (I changed it up to add some variety). I can't recall the exact composition, but if I were to attempt to add to the force I'd try Mechanicus Standard Grey or Ceramite White as the basecoat, perhaps some Fenrisian Grey or Dawnstone over the Mechanicus Standard to lighten it up a touch, and then a wash with Drakenhof Nightshade to add the colour.
I'm sorry I can't be more precise - I really am terrible, I recently had to go through half a dozen test models because I'd forgotten the mix I use for my elf cloaks. I need to start recording this stuff down when I use it - my memory is like a sieve with the mesh bit removed.
It does end up looking quite good, and goes well with the ghosts (who are just Nihlakh Oxide over white for a good spooky glow).