It sounds like your marine army has had a lot of thought put into it what with specific helmets for certain things, sounds great.
I'm not actually sure i can visualize your army and make a suggestion on the assault termies colour schemes.
But to try to keep the theme of certain helmes for certain jobs what about using marine helmets on the termies, I saw the marine army in City of death book did this with his army commander rather successfully with a black templars knight like helmet. Maybe sourcing 5 of those helmets. or no helmets and bare heads only to show how they like to look in the eye of their enemy before ripping its heart out.
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