Sorry I don't have any Citadel or Vallejo options for you; my collection of the former is ancient and long
OOP, and I don't own many of the latter because they don't play nicely with my wet palette / climate. Vallejo almost certainly has equivalents for Mage Hunter / Gravedigger. Hopefully someone with a better selection of Vallejo can assist in finding matches.
P3 paint is very similar to old school Citadel should you decide to try them. The range leans towards less saturated / more consistent coverage by buffering with black and/or white in most of their colours, which lines up well with your desired colour scheme. Mage Hunter is actually one of their least saturated options. Cryx Bane Highlight and Cryx Bane Base might also fit in really well; they're brownish-olivish-greenish greys. Very hard to describe, but good neutrals for a grim-dark look. Cryx Base makes a good substitute for black while having more interesting undertones. Cryx Highlight is a middle-of-the-road grungy grey that looks greener when at full opacity, and looks browner, like a greenish khaki, thinned out.
Scanned a bunch of stuff out of my colour book that might help in nailing down a paint scheme.
Good luck with your pile of possibilities!