Covering the underlayer, superimposing layers, and trapping

Printing ink is semitransparent by nature. Hence, for instance, when yellow letters are printed over a blue underlayer, the blue color must be eliminated in just the areas where the letters appear. This process is referred to as undercolor removal.

If the yellow letters were simply printed over the blue underlayer, the colors would blend, resulting in a green color.

Covering the underlayer (left) and overprinting (right)

When the CMYK plate containing the letters is printed over the blue underlayer, care must be taken to prevent any slight skewing of the plate, which would result in a gap between the edge of the letters and the blue underlayer. To handle this, it is necessary to slightly widen each of the CMYK color areas. This serves to prevent gaps (through which the white background would show), and is referred to as trapping.

Unless trapping is used, the white underlayer will appear when the plate is skewed

Compare:Understanding Color/The three primary colors

Compare:Understanding Color/How are colors reproduced in printing?

Compare:Glossary/CMYK