I'll answer a couple things at once.
The 62 plates were almost certainly portions of the actual ones used to print the cards.
All the following is for "modern" offset lithography, which is most commercial color printing from roughly the 1920's to 1990's. There isn't much change today other than how the plates are produced.
The plates mount to a roller in the press, and the coating retains water. The ink is oil based so the water retained on part of the plate rejects it while the portions intended to print don't.
So the plate passes first a roller that wets it, then one that inks it. (There are several rollers to spread the ink smoothly before it gets to the final inking roller. ) The next roller is the "blanket" a roller covered with a rubber sheet that the plate prints to. Positive image on the plate prints reversed on the blanket. Then the blanket rolls against the paper which passes between it and the impression cylinder.
Typically you want to run the lightest colors first. Yellow, then Cyan, magenta and finally black. For a two color thing like a typical topps back you'd run the color first then black. Yellow won't typically be opaque enough to cover black. -- It's interesting to note that some older cards like T205s didn't finish with black. On those you can often see the gold border covering the black frame lines. And I'm fairly sure some 1981 fleer were run with yellow after the black on the back.
When the plate is removed there's still some ink sticking to the parts that hold ink. So the yellow plate looks yellow, the magenta plate pink etc. A number of the modern press plate cards if they're the actual plates have been treated after use to make the image black. The last step in developing a plate is wiping it with a black or brown colored substance to make the areas intended to print show up clearly. It wouldn't be hard at all to make two plates from the same mask, one for production and the other to be cut into cards.
At least one company is using another process, and the plates show the card in mirror image those print directly to the paper.
Steve B
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