As I recall you made that mockup by simply inverting the colour hues?

Why don't you make two versions one with inverted hues the other without. Adjust the 'normal' version to be more red shifted than blue shifted and rotoscope between the two?

That way Obi-Wan will have the correct flesh tones and the right saber colours will reflect off his clothing but Fett will have a helmet exactly like the one you created in your mock-up.

You may have to re-do the saber blade to keep the colour consistent with the rest of the film but it could work (very fiddly though).