Grey component replacement
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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 (decrease saturation), 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 changes the saturation of the resulting color. The most efficient way to change the saturation of a given color while maintaining the same hue angle is to use the Key (Black) component. The act of substituting a quantity of black for a quantity of the greying agent is known as "Grey component replacement."
In grey component replacement (GCR), contrary to under color removal, the CMY values that add to grey all along the tone scale can be replaced with black ink. Gray component replacement only adds black to the CMY equivalent of what would have printed as a grey.
- Advantage: Black ink can be less costly than colored inks.
- Advantage: The resulting output is less susceptible to changes in the printing variables since you are not continually trying to balance as much C, M, and Y.
- Disadvantage: As a result of impurities in the C, M and Y primaries, some colors can appear dirty or may shift.
