About knockouts and overprinting
If you are printing your publication through a commercial printing service, you might need to work with knockout and overprint settings. It's best to have a print professional adjust the knockout and overprint settings for your publication.
When colors print on top of each other, they can mix to create an undesired color. For example, if a blue circle is printed on top of a red rectangle, the overlapping colors mix to create purple. Knocking out the colors beneath selected objects prevents the overlap problem--the foreground colors print directly on the paper instead of on top of other colors.
The opposite of knocking out is overprinting, where text and objects in the foreground print directly on top of any background objects.
Overprinting works best for objects and text that are black or a dark color.
For process-color and spot-color publications, Microsoft Publisher automatically determines when to knock out and overprint text and objects. By default, Publisher overprints text below 24 points, black lines, and all patterns and imported graphics that are recolored to black in Publisher.