Grey component replacement
Within the CMY color space, a range of colors can be achieved by combining the three primaries. This combination in its turn can be thought of as a hue component and a grey component. If the grey component is replaced by black ink, the same color is being achieved by using two primaries and black. The act of substituting a quantity of black for the grey component is known as "Grey component replacement".
In grey component replacement, contrary to under color removal, the CMY values that add to grey all along the tone scale can be replaced with black ink. UCR only adds black to the CMY equivalent of what would have printed as a grey or near-grey.
