Hey Ga.modeler,
Don't forget that the M108 usually had the semi gloss OD.. the old guys I knew when I first joined told me they used to 'oil' the dull paint to make it shine, but I don't know how true that is. Sounds like something they'd do though! When I went to Fort Sill, they were still training on M109 A- nothings. With a locally approved two part color scheme of OD and tan... No M108's in 1975 either. 105mm was all towed.
Modelmaster Dark Green with semi-gloss over it is a pretty close match to the origninal color, which really made those old white stars stand out.
Ron.