A good starting point for understanding the CMY colour model is trichromatic colour theory.
Trichromatic colour theory explains the underlying physiological basis for the subjective experience of colour.
Trichromatic colour theory and its precursors have established that there are three types of cone cells (recognised by the initials L, M and S) in the human eye that carry out the initial stage of colour processing that ultimately produces the world of colours we see around us:
L = Long (500–700 nm)
M = Medium (440 – 670 nm)
S = Short (380 – 540 nm)
Trichromatic colour theory also states that three monochromatic light sources, one red, one green, and one blue, when mixed together in different proportions, can stimulate the L, M, and S cones to produce the perception of any colour within the visible spectrum.
All colour models, such as the RGB and CMY models, have their foundations rooted in the trichromatic principles of human vision