I had a brief email conversation with Owen L. Conner, Curator for Uniforms & Heraldry at the National Museum of the Marine Corps about this subject. He confirmed a couple of things for me.
1) USMC Forest Green during WWI is not the same color as that used in WWII, it is close however and in 15mm or even 28mm you aren't really going to be able to tell the difference. Mr. Conner described the WWI uniform as Emerald/Field Gray Green and the WWII uniform as Forest Green mixed with a bit of Olive Drab.
2) Giving the same specifications to two different manufacturers of WWI uniforms was likely going to produce two different shades of Forest Green. So even a Marine stepping right off the boat into WWI France could already be in a mismatched uniform.
3) The museum refers calls uniforms displaying the divisional badges as parade ground uniforms. The badges did not appear on most uniforms and helmets until after the armistice. Now obviously some must have existed on uniforms prior to this but it would be considered rare. Apparently they were sometimes applied to the gasmask bag as well.
4) Reports of the use of the Eagle, Globe and Anchor on the helmet are overblown. It certainly did happen but its primarily associated with the period between WWI and WWII.
I have a few pictures that Mr Connor was kind enough to send along with our emails which I'll share with you below. He also recommends "US Marine Corps in World War I, 1917-1918" (Men-At-Arms Series, 327) by Mark Henry, an excellent and affordable resource (which I already have, nice to have it confirmed by an expert though). So on to the pictures:
First up is a WWI Army Jacket, which is much greener that I thought it would be.
Here is a Navy Corpsman's gasmask bag. Note the Divisional symbol which was applied after the armistice, at least according to Marine Corps Records
And we follow that up with an example of non-matching uniform colors probably coming from two different manufacturers or possible even different lots of cloth from the same manufacturer.
So there we have it, a brief look at the colors of WWI Marine Corps uniforms.