How does CMYK work?
Because process color ink pigments are imperfect, pure black cannot be achieved by overprinting CMY inks.
Consequently, black (K) ink is introduced in addition to, or in substitution for, CMY inks. The combined value of all CMYK inks for a particular area or object cannot exceed a specified amount, or ink may not transfer effectively and printed sheets may not dry properly.
This specified amount, referred to as Total Area Coverage (TAC), typically is limited to 300% for offset lithography using coated paper. Compensation for TAC limitation is accomplished during the separation process, by which RGB color is converted to CMYK.
The images below show all color print processes seperate: cyan, magenta, yellow & key (black).
CMYK color seperations
CMY & CMYK color combinations
The image to the left shows the image when the primary CMY inks are combined. The image is lacking the shadow area details, and looks "washed out". To correct this the color Black is introduced to the image
In the image to the right the Black (key) has been incorporated into this image, and it now is ready to go to press!
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