I would think that black would probably look the best. I've read that Stormtroopers generally use this color because if light comes up from below the neckline (remember of course they are highly polished white) that it has a tendency to "bleed" through the lenses. So it cuts down on visibility from folks outside the helmet.
I painted mine in a flat black and sealed it with an enamel so it won't get all over my head when I sweat, and it looks pretty clean. The other option is to go full out and build an interior based on the Riddell mini-helmet. Anyway, that's my idea.
Geoff