CMYK belongs to the subtractive color models. To simplify, this means that if all colors are 100 percent active, the resulting color is black; if all colors are absent, i.e., at zero percent, (white) paper remains white. The intensity of different color nuances is comparable to mixing colors using a color box, for example, blue mixed with yellow results in green. Depending on the amount of water and color, a different shade of green is created.
The important difference to RGB: The absence of all three colors (RGB = 0, 0, 0) ensures black in the RGB color space. If red, green, and blue are each at their maximum value of 255, white is displayed. Being an additive color model, RGB is the exact opposite of CMYK.
In practice, it has been found that printing 100 percent cyan, magenta, and yellow results in a very dark shade of brown. In the figure above, this can be seen in the color field CMYK = 90/90/90/0. To create a true black, an additional component is needed. That is why the color black (K) has been included in the color space. At the same time, the addition of black made it easier to scale the other three colors. In theory, it’s possible to display up to four billion different color tones with CMYK colors.