Really, it depends on which Cryx scheme you're trying to replicate.
The
new one?
The
remix one?
Or the really, really old metallic scheme?
Personally, I find that the easiest way to replicate the Bane colors is to
use the Bane colors. There's no good analogue in the
GW/VGC ranges (neither includes a green-gray), but if I had to mix it, I'd say adding black (or Codex Gray, for the higlight) to Rotting Flesh would be a good start. The P3 range tends to have a lot of colors that aren't easily replicated by other manufacturers (other than VMC, if you have access to the full range).