Cyan, Magenta and Yellow are "subtractive colors".
If we print cyan, magenta and yellow inks on white paper,
they absorb the light shining on the page. Since our eyes
receive no reflected light from the paper, we perceive black.
The printing world operates in subtractive color, or CMYK
mode. In practice, printing subtractive inks may contain
impurities that prevent them from absorbing light perfectly.
They do a pretty good job with light colors, but when we
add them all together, they produce a murky brown rather
than black. In order to get decent dark colors, black ink
is added in increasing proportions, as the color gets darker
and darker. This is the "K" component in
CMYK printing. "K" is
used to indicate black instead of a "B" to avoid
possible confusion over Blue ink.