Adobe Acrobat 7 Professional - Manual (Page 683)

CMYK model
Whereas the RGB model depends on a light source to create color, the CMYK model is
based on the light-absorbing quality of ink printed on paper. As white light strikes
translucent inks, a portion of the spectrum is absorbed. Color that is not absorbed is
reflected back to your eye.
Combining pure cyan (C), magenta (M), and yellow (Y) pigments would result in black
by absorbing, or subtracting, all colors. For this reason they are called subtractive colors.
Black (K) ink is added for better shadow density. (The letter K came into use because
black is the key color for registering other colors, and because the letter B also stands for
blue.) Combining these inks to reproduce color is called four-color process printing.
Subtractive color (CMYK)