CMY Color Cube:

In the printing trade this model is frequently called as the CMYK model in which the K stands for black. The purpose for black is that, although theoretically the correct mixture of cyan, magenta, and yellow ink must absorb all the primary colours and print as black, the best that may be achieved in practice is a muddy brown. Thus, printers like the Hewlett-Packard Paint Jet have a separate cartridge for black ink in addition to the cyan, magenta, and yellow ink cartridge(s).

The CMY model may also be represented as a colour cube as illustrated in Figure

Figure: The CMY Color Cube

Each corner is labelled with its (c, m, y) coordinates. Note down that the RGB colour cube is transformed into a CMY colour cube by interchanging colours across the major diagonals.