Kubelka–Munk theory was devised in 1931 to compute paint-film opacity, not general color

Kubelka–Munk (K–M) theory, published in 1931 by the Czech chemical engineer Paul Kubelka and Franz Munk, is a semi-empirical two-flux model of diffuse light travelling through a scattering, absorbing medium. Its founding question was narrowly industrial, not a general theory of color: the original paper addressed "the question of how the color of a substrate is changed by the application of a coat of paint of specified composition and thickness, and especially the thickness of paint needed to obscure the substrate." In other words, the problem it was built to solve was paint hiding power — how thick a coat must be to obscure what is under it.

The model reduces a material's optical behaviour to two coefficients, an absorption coefficient K and a scattering coefficient S, whose ratio K/S maps to diffuse reflectance. That compact abstraction is why the same equations later travelled far outside paint: they are reused, essentially unchanged, in paper and textile manufacturing, in semiconductor band-gap estimation (Tauc plots), and — the load-bearing point of this cluster — in claim-kubelka-munk-reflectance-theory-grades-meat-freshness-via-myoglobin and claim-kubelka-munk-theory-computes-dental-shade-matching. K–M sits one optical-physics layer beneath the perceptual color spaces the vault already tracks: where claim-cielab-designed-to-remedy-ciexyz-perceptual-nonuniformity re-maps tristimulus values to human perception, K–M models the underlying physics of light in the medium that produces those values in the first place.

This is a historical/definitional claim resting on a single Tier-4 encyclopedia source, so it is held at seedling. The primary home is Kubelka & Munk's original 1931 paper (Zeitschrift für technische Physik 12:593–601); confirming the framing and date against it is tracked in question-verify-kubelka-munk-1931-original-paper-paint-opacity.