When a PostScript file is printed on a composite colour printer, elements printed last overprint previous elements and knock out where they overlap; the ink is not mixed to produce a third colour. Older elements are totally obscured by newer elements.

When producing separations there is a minor difference in that elements are effectively printed on four separate plates that are independent from each other. These are recombined at printing time and the colour details can interrelate in a number of ways.

A good example of this can be shown when looking at the difference between printing 100% black and 50% gray elements. For example, if three shapes are printed, the first is 40% cyan, the second two, which overlap the first shape, are 100% black and 50% gray.

When printed, the black overprints the cyan. This is because the cyan component is not visible through the 100% solid black ink, and knocking out the cyan could produce problems if there is a slight misalignment between the plates when printed on a press; a white slither could appear between the two areas.

In the case of the 50% gray, a white hole (or knockout) is left in the cyan for the gray to be placed. If the knockout were not performed on the cyan plate, the gray ink would mix with the cyan, producing an area of darker cyan.

Overprint works for all colours. However, colour on a plate only overprints other plates on which it has no ink. For example, a red colour is made up of yellow and magenta. Thus, it can only overprint the black and cyan plates along with any other spot plates defined by the user.

The percentage of the overprint is the threshold at which the colour overprints. For example, black only overprints at 100%. If a colour is always required to overprint, the threshold should be set to 0%.

The Allow Knockout option for spot colour enables a spot colour to be affected by the overprint of other colours.