CMYK, or 'four color printing', is what is called a 'subtractive' color model, because it starts with white - such as paper - which is the combination of all colors, and filters of subtracts out those colors not wanted by the application of colored dyes or inks.

Putting, for instance, a cyan lens in front of a white light, or cyan ink on a sheet of white paper, will prevent other wavelengths of light from passing through the lens or reflecting back from the paper. Using combinations of these ink colors in varying amounts will produce the various colors needed for a picture or graphic. Equal amounts of yellow and cyan, for instance, produce a standard, medium green.

Key black is added to the four color printing process because the combination of cyan, magenta, and yellow in practice produce a 'black' that looks more like a dark muddy brown.

RGB is called an 'additive' color model because it starts with the absence of light or color - black - as one would find in a television screen or computer monitor with no power, and adds light in colors as needed.

As with CMYK (and other color models) the combination of these colors in varying amounts creates the secondary and tertiary colors desired for more complex images.