If you are printing your publication through a commercial printing service, you might need to work with knockout (The process of eliminating background colour behind an object in the foreground.) and overprint (To print an element of one colour over one of another colour without removing, or knocking out, the material underneath.) settings. It's best to have a print professional adjust the knockout and overprint settings for your publication.

When colours print on top of each other, they can mix to create an undesired colour. For example, if a blue circle is printed on top of a red rectangle, the overlapping colors mix to create purple. Knocking out the colours beneath selected objects prevents the overlap problem--the foreground colours print directly on the paper instead of on top of other colours.

Knockout 1

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 colour.

Knockout 2

Another factor that will influence the visibility of the traps is the direction of the trap. The decision which color should be spread or choked is usually decided upon the relative luminance of the colors in question. The ‘lighter’ color should always be spread into the darker. Again this reflects the way the human eye perceives color: since the darker colors define the shapes we see, distortion of the lighter color will result in less visible distortion overall. The ‘lightness’ or ‘darkness’ of a color is usually defined as its ‘neutral density’.

More on this subject from Quark

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