Register Marks and Overprinting
Let's look at another example of a three color book. Arnold Lobel used a technique similar to Mr. de Paola's to create the illustrations for Judith Viorst's book I'll Fix Anthony. Again, there is a continuous tone key plate and two flat tint overlays. The overlays here are executed on thin paper rather than acetate, but the same principles apply.
In any artwork utilizing overlays you will notice markings, usually around the edges of all of the pieces, that look like crossed circles: ⊕. These are known as register marks. The artist places these marks on the key plate and each overlay, ideally before they begin the illustration. These marks serve as a guide for the printer; as long the register marks are lined up correctly the different colors should print in the proper alignment. When this fails to happen a book is said to be "out of register". Proper register is especially important when colors are overprinted.1
Ariane Dewey and Paul Zelinsky explain register marks
One other thing to notice here is that the overlays are drawn in black and shades of grey rather than just solid black. Halftone screens will be used when photographing these overlays in order to transfer the images to film. This will allow the plates to print the images in a way that appears to be continuous tone, just like the art, with tiny halftone dots where the color is light and large, even overlapping halftone dots where the color approaches solid coverage. This allows the artist to have multiple shades of ink printed in the final book.
Another important detail here is that while there are only three color separations--black, red, and yellow--the final printed book has four colors--black, red, yellow, and orange. This is possible because the inks used in printing are transparent and can be combined to create new colors when they overlap. This combination of inks in different tones to create new colors is called overprinting.2 If you look closely at the separations and compare them to the printed pages you will find that areas of orange in the book were printed by combining a lighter tone of red and a darker tone of yellow.