In four-color printing, (or more) 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.
Why?
With current ink technology, the total CMYK ink in the shdows refuses to stick after it reaches the dark shadows. It begins to peel off. In order to prevent this, printers developed a system called UCR, in which the neutral shadows which would have normally been produced by printing the primaries cyan, magenta, and yellow in the shadows (high ink coverage) is replaced with black .
- Advantages: solves the ink not sticking problem (called ink trapping)
- Advantages: black ink is cheaper
- Possible disadvantages:Black ink by itself in a shadow is not dark enough, so they'd better put back a little of the CMY in the shadows (Called Undercolor Addition UCA) to make a pleasiing reproduction
