Color printing requires individual plates for each color of ink. In order to reproduce full color artwork the colors must be separated. Often this was done by the camera person using a special technique to photograph the art and create a different piece of film for each color. This technique, know as camera separation, was used to reproduce photographs, paintings, and many other types of continuous tone, full color copy.
In many cases, though, the artist was asked to do the work of separating colors. This technique, known as preseparation, involved artwork created with the printing process in mind. It was widely used in children's book publishing as a way of reducing costs. Camera separated art required extensive attention by the camera person, whose labor was expensive. Preseparated art usually involved the creation of separate pieces of copy for each color of ink to be used, which minimized the work of the camera person.1
Take a look at these pages from Dahlov Ipcar's book The Wonderful Egg.
There are two colors of ink in this image: green and black (in printing, black is considered a color). That means that these pages were printed using two printing plates. In this case, the artist created two separate pieces of copy, one for each color of ink.
These pieces are known as color separations. If a printing plate is like a stamp, the color separation served to tell the camera person: "make a stamp like this". Because a different plate is needed for each color of ink, the artist created a color separation for each plate. Color separations could be either line copy or continuous tone copy.
The camera person would photograph each separation to capture it on film that could be used in a chemical process to transfer the image onto the plate. The black and white film used by the camera person could pick up some colors well but had a hard time with others: red, orange, yellow and green photographed about the same as black, while blue and violet photographed almost the same as white.2
In this case, the artist used both black and green ink to make the green color separation. The camera person and platemaker did not maintain the distinction between these two colors. Instead, they treated them both as solid areas to form a single printing plate. This plate was used to print the green portions in the final book, not because the artist's color separation had some green in it, but because the artist indicated that the plate was to be inked with green ink. In most cases, artists simply used black or shades of grey to make their color separations because they picked up well on camera, which limited the work and responsibility of the camera person. This required a difficult skill from the illustrator: the ability to think in color but work in black and grey.3
1. Gates, Frieda. How to Write, Illustrate, and Design Children's Books. Monsey, N.Y.: Lloyd-Simone Pub., 1986. Print. 85-86 ↩
2. Hackleman, Charles W. Commercial Engraving and Printing. 2nd ed. Indianapolis, Ind.: Commercial Engraving, 1924. Print. 19 ↩
3. Stevens, William J., and John A. McKinven. How to Prepare Art and Copy for Offset Lithography. Maywood, N.J.: Dorval Publishing, 1948. Print. 87 ↩