I've heard a lot of people suggest window tint, but I've never seen anyone use it. If you're looking to build the visor from scratch anyway, I would recommend using a method like this:
http://405th.com/forums/index.php?showtopic=18292
that way you can get the most accurate visor shape as possible. It isn't as shiny, but you can probably get a color that's closer to the helmets ingame.
Or there's the tried-and-true motorcycle helmet visor method. I just got a gold iridium visor off of ebay for 15 dollars, so I highly recommend it.