In four-color printing (or more), under color addition (UCA) is a technique for darkening areas of the printed image by adding colored inks. It is meant to achieve the same results as under color removal

Under color removal

Ideally, CMY without black should complete the printing process; however, sufficiently pure CMY ink has not been developed, and for this reason, black ink is used to add details and darkness in shadowed areas...

, but from a different starting position.

Under color removal replaces colored inks in selected areas with black ink to achieve a darker appearance, whereas with under color addition, small portions of the three colors are added to these areas instead of, or in addition to, using black ink.

UCA is used to reduce the speckle effects of dithering, especially with very light colors. Without UCA, a very light grey would consist of infrequent black specks that might be individually noticeable. With UCA, the specks are lighter and 3x more numerous.

Printer drivers sometimes make the use of UCA somewhat configurable, disabling it when set to a mode intended to save ink.

Graphic designers (or anyone else creating artwork for print output) are cautioned to be wary of using UCA. Used wrongly, the effects can be disastrous, rendering poorly printed images and yielding the problems associated with rich black

Rich black

Rich black, in printing, is an ink mixture of solid black over one or more of the other CMYK colors, resulting in a darker tone than black ink alone generates in a printing process....

. On the other hand, UCA can breathe life into a flat image, adding depth and richness to the shadows.

UCA is generally best used in conjunction with under color removal

Under color removal

Ideally, CMY without black should complete the printing process; however, sufficiently pure CMY ink has not been developed, and for this reason, black ink is used to add details and darkness in shadowed areas...

- in order to remove some, but not all CMY ink from black areas.