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A fair generalization is that many full-frame vehicles have semi-gloss black frames, with the floor portions, being part of the body shell, painted body color. This holds true for older cars as well as recent pickups. Unibody cars often have the underside painted body color, again as it's a welded part of the body structure, and in more recent cost-conscious times, to save paint the undersides of the unibody shell will show a red, grey, black or green-gray primer, with overspray of the main body color around the edges.
This generalization works only for cars manufactured in the robotic-painting era. In the decades before our modern era, car bodies for "body on frame" construction were painted by humans, using hand-held spray guns, the car bodies almost universally being moved into the spray painting booths in the factories on steel carts, the bodies themselves being positioned perhaps a foot or so above floor level. Depending on the car, and the simplicity (or complexity!) of it's styling, a crew of 2 or 3 painters did the work of adding the body color. Of course, prior to this step, from at least the middle 1950's forward, newly welded up car bodies were dipped in vats of primer (and by the 60's, rust inhibitors--such as they worked) first, baked dry, and then sanded smooth. This setup precluded painting the undersides of car bodies with color, but the painters, in "ducking down" to coat the underside of rocker panels, and when painting the firewall, did leave a swath of overspray on the outer sides and across the front of, the floorboards.
Automobile frames pretty much have been painted a semi-gloss black (and a cheap paint at that) before hitting the assembly line, as were other painted components, which meant that bolts and nuts (along with little brackets and clips for holding wiring, brake and fuel lines, and the like were raw metal (generally cadmium plated), and of course, gas tanks were galvanized sheet steel, with unpainted, raw steel exhaust systems.
The above painting process was, I believe, pretty much the case with unibody cars as well, certainly well into the 1970's (it sure was the case with my brand-new 1970 Barracuda!). One thing about unibody cars however: Depending upon whether or not subframing was welded onto the body shell (AMC, along with Chrysler and Ford's Falcon and Mustang) or bolted on (front subframing for certainly 1967-later Camaro's and Firebirds, along with the Nova). Bolt on front subframes more often than not were painted black, where welded on subframing generally got the same color as the outer surfaces of the body shell.
The first car I ever owned that had its undersides painted body color was my 1986 Plymouth Voyager, and I would suspect that this is very much the case today.