CMYK

The CMYK color model, in contrast to the RGB model, is a representation of the transmission/absorption and reflective/absorption sources of light – also known as a subtractive color model. This color model describes the printing process which utilizes mediums that absorb certain wavelengths and transmit or reflect other wavelengths. Examples include paper (reflective) and slides (transmissive).

In the CMYK model the letters represent:

C - C stands for Cyan which is the color that is produced when red is absorbed by the medium and green and blue are transmitted/reflected.

M - M stands for Magenta which is the color that is produced when green is absorbed and red and blue are transmitted/reflected.

Y - Y stands for Yellow which is the color that is produced when blue is absorbed and green and red are transmitted/reflected.

K - K stands for Black (B was not used since it is used in the RGB model). Black ink was added to darken the ink produced by the CMY colors since the output produced by all three of them was a brown instead of a pure black.

For this reason, it is common that the RGB color model will be used by computer screen manufacturers as they are concerned with the emission of light. Conversely, commercial printers will discuss colors using the CMYK model which represent the colors that are depicted on paper or other reflective mediums.

Portions of this post were adapted from the book:
Fundaments of Color - Shade Matching and Communication in Esthetic Dentistry published by Quintessence Publishing Co. Inc and written by Dr. Stephen J. Chu, Dr. Alessandro Devigus, Dr. Rade D. Paravina and Mr. Adam J. Mieleszko. and the Wikipedia article: http://en.wikipedia.org/wiki/CMYK_color_model