I'm not so sure about "dark" green, but green.
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I have the Osprey book, which seems to indicate the helmet is about the same color as the uniform.
I don't think that's correct. Looking at various online pics and vids the picture is pretty consistent. The color of the helmet was green.
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Most pics are black and white, so the color is hard to see directly. What is clear is that the uniform itself tended to fade in color quite a bit, and the helmet seemed to retain it's base color.
Here is a pic of an actual surplus WW2 Romanian helmet I pulled from eBay some time ago.
In my conversations on various fora I concluded the following. Before adopting the Dutch helmet, the Romanian army used the French Adrian helmet. (It was still in service in WW2 in limited numbers.) These helmets came from France in the same color that the French army used. When they changed to the Dutch helmet they retained the same color as with the French Adrian helmet.
So for my Romanian force I used "French Khaki" by Polly-S. It is a reasonably rich green, but also not a very dark green. I then use more typical khaki colors (with more tan or brown) for the rest of the uniform, using a lighter shade for the tunic (cotton, tended to fade faster in service) than the trousers (wool, faded but not as fast).
Here is most of a platoon. I have a full company … 3 platoons and a support platoon, with HQ and observers and commo teams and medical section and various gun crews.
Here is a closer look at one of the squad stands. I only use 4 figures per stand, even though a Romanian squad was pretty big … typically 14 soldiers in 1941/42, and maybe 10 in 1943/44/45.
Here is a look at a mortar team. I think the Tamiya color I used for the mortar is called German Uniform Grey. It is actually a very green gray, and as most Tamiya acrylics have a bit of a gloss to them it works well for metal stuff. But I don't think it would look right at all for the helmets.
Just my thinking. Your mileage may vary.
-Mark
(aka: Mk 1)