I'm just speculating here, but that looks like a metallic finish. If so, it is probably a darker shade metal, like copper or bronze, or else maybe a gun metal color or steel. The sheen on the body tube leads me to believe that it is covered with a metallic paint. The drawing of the Voyager in the 1971 FSI catalog has a dark gray body tube, which might indicate that the model in the photo does too. FSI did boast, somewhere in one of their catalogs, that their body tubes could be finished to look like metal, and I suspect that the metallic theme was something that the company was really into for awhile. Both the Orbit and the Nova shown in the same catalog look like they have metallic finishes on their upper body tubes, as does the payload section of the Oso. The wide band below the nose cone of the Viking III also appears to be something like chrome. FSI used to include a chrome stick-on decal of their logo in all of their kits. (I would love to get my hands on one of those decals.) If not a metal, then the Voyager could have a shiny graphite or smoke gray color. (Of course, all colors are represented as grays in black and white photos, but this looks like it really could be a very shiny gray if it isn't a metal.)
There are other photos of rockets in the catalog in which certain parts have bright reflective highlights, such as the nose cone on the Sprint and the body and tube fins of the Viking II, but these look more like glossy paint to me rather than metallic colors. (I suspect that the color of the Viking II is red or burgundy, because reds come out looking very dark in black and white photos.)
I'm struggling to come up with classic, iconic paint schemes for my FSI clones, too. Nearly all of them are flying in just white primer, just like the models in the Reese Industries catalog at Ninfinger! Ghosts, I call them... The photos in the Reese Industries RC-1 catalog all have a rather disturbing, spectral look to them, don't you think?