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Zip-a-tone is/was a self-adhesive plastic film that was pasted onto the art and trimmed to shape.* Duo-Shade is/was a pattern printed into the boards themselves, made visible by painting with special chemicals.

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* Zip, while it produced some good effects, could be a nightmare to work with. As I used to say, back in the day, one could press and buff down the zip as much as humanly possible, yet the sweep of a hand could dislodge it. Perversely, if, when applying it, one happened to touch it to the board in the wrong place, it would instantly bond on a molecular level, making moving it impossible without tearing the paper.

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Yes. When the line is "held" in color, it's a color hold.

With the advent of computer coloring, a lot of colorists have gone quite mad with color holds. Clouds, fire, ice, reflections -- anything and everything gets knocked back with color holds, even when doing so weakens the dramatic impact of the line!!!

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Yes. My understanding is that the traditional materials have pretty much disappeared. Tom Palmer actually makes his own zip now, and I've even done it myself a couple of times, to create a specific effect.