Not Citadel but Vallejo Game Colour Tinny Tin is a good starting point. It's the base for the pad on my Deathwatch Minotaur and looks a decent match to those Thallax:
Just added a couple of layers 70:30 and 50/50 Tinny Tin : Army Painter Gun Metal on top of the base and quick brown wash (Army Painter Strong Tone).