In four color printing (or more), below color additions ( UCA ) is a technique for darkening the printed image area by adding colored ink. It's meant to achieve the same result as under color removal, but from a different starting position.
Under color removal replaces colored ink in selected areas with black ink to achieve a darker appearance, whereas with the addition of color, small parts of the three colors are added to these areas instead of, or in addition, using black ink.
UCA is used to reduce the effects of speckles from dithering, especially with very bright colors. Without UCA, a very bright gray color will consist of rare black spots that may be seen individually. With UCA, the speck is lighter and 3x more.
Printer drivers sometimes make UCA use somewhat configurable, disabling it when set to a mode intended to save ink.
Graphic designers (or anyone who creates artwork for prints) are warned to be careful using UCA. Used wrongly, the effect can be catastrophic, rendering badly-printed images and resulting in problems associated with rich black. On the other hand, UCA can breathe life into flat images, adding depth and richness to the shadows.
UCAs are generally best used in conjunction with bottom color removal - to remove some, but not all CMY ink from black areas.
Source of the article : Wikipedia
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