When designers deal with color, they usually rely on one of two color models: the additive color model, in which individual color waves combine to form white light, or the subtractive color model, in which pigments are used to subtract light waves. Both the traditional artist's palette and the CMYK systems are subtractive color models. On the Web, where we deal with light projection rather than light reflecting off of objects, we use an additive color model called RGB.

Additive color

In the natural world, the light waves that reach our retina are reflected off of objects, but there are other ways to produce color. Stage lights, for example, produce color by projecting white light through colored filters. Computer monitors also use projected light, but in that case the light is produced when electron guns are fired against a phosphor screen. These guns fire electrons in three colors: red, green, and blue. Using only these three colors, monitors can produce a full spectrum. This is known as the RGB color system.

Within the RGB system, designers can also produce a color spectrum by amalgamating these three primary colors. Combining two of the primary colors generates three secondary colors: cyan, magenta, and yellow. As previously noted, adding all three primary colors together produces white light. Thus, an RGB value of 255,255,255 produces white. The complete absence of the three primary colors (RGB: 0,0,0) produces black.

Subtractive color
The inverse of the RGB model is the CMYK model, in which light waves are subtracted to produce the desired color. Since the color of an object is derived from reflected light waves, this system uses three primary colors that each absorbs red, green, or blue light. For instance, if you subtract red light, the remaining green and blue waves produce cyan. The pigment used to subtract red light and reflect green and blue appears cyan. Similarly, print designers use magenta to absorb a percentage of green light and yellow to absorb a percentage of blue light.

At this point, it should be apparent that the primary colors of the CYMK model are the secondary colors of the RGB model, and vice versa. Moreover, if red, green, and blue light combine to produce white light, it stands to reason that cyan, yellow, and magenta pigments should combine to create black, since they should absorb all of the light waves. However, due to the limitations of the pigments and the printing system, the full combination of cyan, yellow, and magenta doesn't quite absorb all light. In practice, it's necessary to add black to the system, hence the K in CMYK.

Color management
These two different methods of reproducing color can be a source of frustration for designers creating graphics for both computer monitors and print material. In addition to difficulties in mapping colors between the additive and subtractive models, the range of colors possible in the RGB and CMYK systems varies significantly. A color management system that converts between the two models based on the output device can solve many problems for cross-medium designers. Color management systems can either be part of an operating system or part of a particular software application.