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

Under color removal

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...

, the CMY values that add to grey all along the tone scale can be replaced with black ink. Grey 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.

See also

• Under color removal
  Under color removal
  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...
  
  
• Four-color printing
• Color printing
  Color printing
  Color printing or Colour printing is the reproduction of an image or text in color...
  
  
• CMYK color model
  CMYK color model
  The CMYK color model is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key...