Metamerism
Metamerism turns up in 2 fields: lighting, paper optics. Here is what it means to the people in each one.
Metamerism in lighting
The effect where two colors match under one light source but not under another, because their spectra differ.
Two surfaces or two light sources with different spectral distributions can produce the same color sensation under one condition and different sensations under another. Illuminant metamerism is why paint or fabric chosen under store lighting looks wrong at home; observer metamerism is why two displays or LED mixes that match for one person do not match for another. Color-critical spaces control it with high-fidelity light sources and spectral matching.
"Upholstery matched the sample in the showroom, but under our 4000 K LEDs it's metamerism, the two fabrics split apart."
Metamerism in paper optics
Two samples matching in color under one light but not under another, because their reflectance curves differ.
Colors match when their tristimulus values agree, which can happen with quite different spectra. Shading a sheet with a different dye set from the customer's master, or matching a brightened sheet to an unbrightened one, often gives a pair that agrees under D65 but splits under shop lighting or incandescent lamps. Color labs check matches under two or three illuminants to catch it before shipping.
"Matches in the light booth on daylight, flips red under the A lamp, classic metamerism from the new dye."