RGB and CMY colour cubes:

The RGB and CMY colour cubes are useful in expressing the transformations among the two colour models. Suppose, for example, that we know a certain ink can be specified by the CMY coordinates, (C, M, Y), and we would like to know what mixture of light, specified as (R, G, B) in the RGB cube, is reflected. Looking at Figure we note down the following 3-D vector relationships:

The inverse transformation may be thought of as solving the following problem: Given light of a certain colour, (R, G, B), reflected from a page illuminated with white light, what mixture of ink, (C, M, Y), is required? Using Figure, we may write a set of equations resembling with White substituted for Black. As, on the RGB colour cube, white contains coordinates (1, 1, 1).