Well, I haven't actually seen the original helmets. I've just spent a lot of time scouring over pictures and owning several casts of high-end helmets that has more preserved detail from the ANH and ESB than you'd believe. Had to sell those, though.
Anyway, the original gunmetal grey is a Rolls Royce color - rumored to be the one called Tudor Grey. I had my eyes set on their Pewter color, but it is too light, but hearing someone from the production say that sometimes darkened the weathering color with black, I'm still hot on the Pewter color, because it has that smooth metallic look that we see on original helmets. The Tudor grey just seems less metallic and more just a slightly grainy grey color to me. But honestly, no one really knows the which the gunmetal color is yet, only that it's a Rolls Royce paint. The black is confirmed as the one called Mason's Black from Rolls Royce. And when you see it, you can tell it's the right color.
The gunmetal color in the pictures seem a little dark and grainy. But I can't say if it is wrong as I need to know if those were taken in natural light or inside and with flash. It just seems a tad too dark.
Can anyone post pictures of the Sideshow shoulder bells? Fron't and side. I've seen them before, but can't really remember.