What about the "K" in CMYK? You could, in theory, mix 100% of cyan, magenta, and yellow to create black. However, you never print 100% of these inks for two reasons: First, ink pigments are imperfect and printing this combination of cyan, magenta and yellow creates muddy brown color instead of a sharp black. In addition, printing too much ink on a particular area of a page, or disc, can oversaturate that area causing the quality of the printing to deteriorate. To achieve fine detail and strong shadows in print, printers use black ink (K) along with cyan, magenta, and yellow inks. Now you've got CMYK.
Spot Colors
Spot colors are printed with premixed inks on a printing press or screen printer. You may choose from among hundreds of different spot-color inks. Each spot color is reproduced using a single printing plate or screen. The Pantone Matching System (PMS) is only one of several popular spot color models used in printing. (dataDisc uses the PMS system) Each PMS number references a unique spot color and these colors can be found in a swatch chart. By using this type of numbering system, folks can communicate exact colors to each other without actually looking at the same samples.
It's important to remember that spot colors may not actually translate to matching process colors. Unlike process printing, which prints dots of the different colors, a spot color is printed at 100% is a solid color and has no dot pattern. A tint is a lightened spot color or process color and is created by printing smaller dots of the base color.