When two colours look like a matching pair under one light source but noticeably different under another.
Metamerism occurs when two surfaces have different spectral reflectance curves that happen to produce the same perceived colour under one illuminant, but diverge under an illuminant with a different spectral power distribution.
Metamerism is not a flaw in the colours themselves — it's a property of how pigments and light interact, and can affect even carefully matched paint batches.
Two 'matching' paint samples from different product lines can look identical in the shop under fluorescent light and visibly different once home under daylight.
Metamerism is a genuine, well-documented reason two nominally matching colours can look wrong together in practice — always check a physical sample in the room it will actually be used in.