i posted this response already in your other tread but i'll put it up here to:
tough decision...i would stick with the metallic colour, but that's mostly because i like it so very much on your other figures. i was thinking, since they're carrying the tzeentch icon, why not give them a sort of greenish magic glow instead of (or in addition to) the reddish lava glow the others have? in that way you a) keep the beatiful metallic b) make the different enough to mark them immidiately out as "special". you could model a sort of small warp-vortexes on the base where the glow is coming from or (and that's a lot more difficult i realise but i have full confidence you're a good enough painter to pull it off without any difficulties) use the icon itself as the origin point of the glow, bathing all marines surrounding it in it's spooky magical glow...
good luck and i'm looking forward to the results, whatever they wil be!