If you hand an experienced oil painter a tube of French Ultramarine and a tube of Cadmium Yellow, their mixture will produce a dull, muddy olive green rather than a luminous spring leaf. This muddiness exposes the fundamental flaw of treating standard "blue" as a true primary color in subtractive systems.
Ultramarine blue carries a distinct reddish bias, meaning it absorbs a portion of the green wavelengths along with the yellow. When introduced to yellow pigment, the red undertone absorbs the very green light that the mixture is supposed to reflect. The result is a neutralized, low-chroma mixture that frustrates novice painters.
To avoid this chromatic degradation, twentieth-century industrial printing abandoned the traditional RYB color wheel in favor of cyan, magenta, and yellow (CMY). Cyan is a greenish-blue that reflects both blue and green while absorbing only red. When cyan is combined with process yellow (which reflects green and red while absorbing blue), the only wavelength neither ink absorbs is pure green. This allows printers and commercial illustrators to achieve an exceptionally vivid spectrum without generating accidental grays.
