Explanation:

CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black). Opposed to this, there is another colour mode RGB that is used mainly for web applications, and when the image is not to be printed.

In additive color models, such as RGB, white is the "additive" combination of all primary colored lights, while black is the absence of light.

In the CMYK model, it is the opposite: white is the natural color of the paper or other background, while black results from a full combination of colored inks.

When CMY “primaries” are combined at full strength, the resulting “secondary” mixtures are red, green, and blue. Mixing all three gives an imperfect black or a perfect grey.

To save cost on ink, and to produce deeper black tones, unsaturated and dark colors are produced by using black ink instead of the combination of cyan, magenta, and yellow.