The quality or state of color independent of brightness; the characteristics of a color defined by hue and saturation alone.
From 'chromatic' + '-ity' (noun suffix for qualities/states). Became standard in colorimetry and color science in the 20th century as precise color measurement became essential.
Chromaticity is why you can have the same 'red' at different brightness levels—the chromaticity stays identical while only the brightness (luminance) changes; this is crucial for display technology and color matching.
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