Cyan was selected as a primary printing color because it's the most effective single pigment for absorbing red light while reflecting both blue and green wavelengths. Its absorption spectrum is remarkably clean, with minimal unwanted absorption in other parts of the spectrum. This selective absorption makes it ideal for subtractive color mixing - when layered with other inks, it maintains high saturation without creating muddy colors. Its molecular structure also allows for stable pigment formulations that resist fading.
Modern cyan inks are typically copper-based phthalocyanine pigments, chosen for their excellent lightfastness and intensity. The ancient Greeks called this color "kuanos," originally referring to dark blue enamel work, though modern process cyan is standardized at much brighter values.