CMY model

The CMY colour model stands for cyan, magenta, and yellow as primary colours in subtractive colour theory. It derives from the CMYK model used in printing. It should also displace the RYB model for subtractive painting.

(Image embedded from Wikipedia on 2 January 2011)

The CMY model can actually be considered as the "modern" version of the RYB model. Yet, because black is left out, some artists may feel a need to select related hues more appropriate to their painting than the middle-of-the-range CMY. For example, some artists may prefer a deeper blue (eg phtalo blue) instead of cyan, in order to mix darker violets or produce deeper tones.

Want to know more?

Handprint - "Primary" triad palette
Bruce MacEvoy discusses a CMY palette in this page. It may be a bit difficult to follow at times, but it offers a basic description and criticism to the palette and, by extension, to the underlying model.