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 4 or more color printing, under color removal (UCR)is the process of eliminating amounts of yellow, magenta, and cyan that would have added to a dark neutral (black) and replacing them with black ink during the color separation process.
With current ink technology, the total CMYK ink in the shadows refuses to stick after it reaches the dark shadows, and begins to peel off. To prevent this, printers developed a system called UCR, in which neutral shadows - which would have normally been produced by printing the primaries cyan, magenta, and yellow (high ink coverage) - are replaced with black.
There is no universal rule for under color removal. The amount required will depend on the printing press, paper, and ink in use.
UCR is generally not recommended for use in printing, due to its tendency to produce dull-looking images and artwork. The main exception to this rule is that where working in newsprint, UCR is the best way to avoid the associated ink limit and registration issues. UCR is also recommended for some specific paper stocks, depending on the coloration and texture of the paper.