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