Up until the mid-eighties, many books were illustrated using a technique known as preseparated color (or color separations, as I called it). While there are and were illustrators who enjoyed this process, I was not among them.  

Here's an example of the process from Go To Sleep, Nicholas Joe, written by Marjorie Sharmat.
Above is the final printed illustration scanned from the book. I used 4 overlays, each representing 1 color: Red, yellow, blue, or black. What you see above are the 4 colors merged as one.
All the illustrations begin with the line art, in this case drawn in India ink with a Rapiograph pen. Many book illustrations are of styles that have no line. They would go right to the "painting" of the overlays.

Note the register marks in the upper and lower corners. These will be used to line up the 4 color overlays.
That's a lightbox on the right. It's used for tracing, and for Nicholas Joe, it was used to lightly trace the black lines to use as guides for the color plates. (Shown is from Pigs to the Rescue)
This is the color chart I used for Talester the LizardThe Talking Tree, Or Don't Believe Everything You Hear, and Go To Sleep, Nicholas Joe. I spent a lot of time staring at those colors and can't bring myself to throw it out. Anyway, this shows what the varying percentages of each color looks like when combined with another. For example, I find the green I want on the chart. I see that it is made with 40% yellow and 60% blue. And now it's on to the overlays...
Above to the left is the red overlay. To the right is yellow. The register marks in the corners are punched with a hole puncher. They line it up to the line art. On the yellow page, notice how the black in the curtains are, on a darkness scale of 0% to 100%, close to 100%. On the left red page, it's about 60%. If you scroll up to the finished illustration, you'll see the orange it turned out to be.
The blue overlay is on the left - black on the right. You'll see there is no blue or black in the curtains. The black was mostly used for shading. Solid black would be part of the line art.

So this is what got shipped off to the printer where the separated colors are brought together as one. However, it wouldn't go to final printing until the artist and art director had the opportunity to first inspect samples, called proofs. While there could be no changes to the art at this point, there is the opportunity for the printer to, for example, tweak up yellow 5% or knock down the blue 10%.

This was an interesting process, but a lot of work. I am glad to have tried my hand at it. But I am also glad that I don't ever have to do it again.
What are preseparated colors?