Metamerism (color)

Colors that match under one light source but differ under another, because their surfaces reflect different spectral distributions that produce the same tristimulus values.

Trichromatic theory: Vision model holding that color arises from three receptor types with broad, overlapping spectral sensitivities. Three broad filters map infinitely many spectra onto only three numbers, making distinct spectra collide.
Color matching: The industrial practice of formulating a material's color to match a standard sample. Metamerism is the central failure mode a colorist must design formulations to avoid.
Grassmann's laws: Hermann Grassmann's 1853 rules stating how color matches combine additively. They give metamerism its formal footing: matched colors remain matched when mixed identically.
Color constancy: The visual system's tendency to keep perceived object colors stable as illumination changes. Constancy fights illuminant shifts in perception while metamerism describes matches that break under them.
Colorimetry: The quantitative science of measuring and specifying color using standardized observers and illuminants. Metamerism is the founding fact of colorimetry: only three numbers survive from any spectrum.
Spectral power distribution: The function giving a light source's radiated power at each wavelength across the visible range. A metamer is defined by differing reflectance spectra that yield identical tristimulus values.
Color rendering index: A 0-100 measure of how faithfully a light source renders the colors of objects compared to a reference illuminant. Poor color rendering sources exaggerate metamerism, so the index gauges that risk.
Maxwell's color experiments: James Clerk Maxwell's 1850s spinning-disk and spectroscope work establishing quantitative color mixing. Maxwell demonstrated many spectral lights matching a three-primary mixture, an early metamerism showcase.
Observer metamerism: Metamerism arising because individual observers' color matching functions differ from the standard observer. A named subtype: two people can disagree about a match that one calls perfect.
Three-color displays: Screens built from red, green and blue emitters that synthesize colors by metamerism. Every display color is a metamer of the real-world color it is meant to reproduce.

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