OD is the primary color of that bridge (or any US Army bridge for that matter), but the pontoons have wood decks that could show massive amounts of weathering on them, ie: paint scuffed, chipped, worn off, etc, so wood would show through, and the bridge sections would show a lot of steel (unless there's been a bunch of recent traffic on a rainy day, then you got mud) through the decks and a lot of chipping on the panels... Those things are HEAVY and get banged around a LOT... Everything is moved via manpower and a truck winch..
Best thing you can do is to vary your OD shades among the three main componets and let the weathering do the rest... BTW, there's no rubber on that kind of pontoon bridge..