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

Yellow

Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...

, magenta

Magenta

Magenta is a color evoked by light stronger in blue and red wavelengths than in yellowish-green wavelengths . In light experiments, magenta can be produced by removing the lime-green wavelengths from white light...

, and cyan

Cyan

Cyan from , transliterated: kýanos, meaning "dark blue substance") may be used as the name of any of a number of colors in the blue/green range of the spectrum. In reference to the visible spectrum cyan is used to refer to the color obtained by mixing equal amounts of green and blue light or the...

 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.

Advantages: solves the ink not sticking problem

Advantages: black ink is cheaper

Possible disadvantages: Black ink by itself in a shadow may not be not dark enough, so CMY colors are added (called Under color addition

Under color addition

In four-color printing , under color addition is a technique for darkening areas of the printed image by adding colored inks...

 or UCA) to make a more accurate reproduction

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

Newsprint

Newsprint is a low-cost, non-archival paper most commonly used to print newspapers, and other publications and advertising material. It usually has an off-white cast and distinctive feel. It is designed for use in printing presses that employ a long web of paper rather than individual sheets of...

, 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.

See also

• Grey component replacement
  Grey component replacement
  Within the CMY color space, any hue angle can be achieved by combining two of the three primaries. The intention of the third color is to move that hue towards grey , and is known as the greying agent. However, as that greying agent has an inherent hue of its own, it also shifts the hue as it...
  
  
• Four color printing
• 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....