My first books were created using pencil on mylar overlays. They called it “pre-separation.” The burden fell on the artist to conceive of the color and create it using only grey and black on separate overlays. The colors loaded onto the print presses and the final color combinations were first seen as the sheets came off the press. I have this original artwork on our wall. It’s the “black plate” for the cover of Cross-Country Cat.
You can see the shadow was done in an overlay called the “blue plate”.