Complex sentence: 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.
Split into simpler sentences: 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.