What is metamerism?

Metamerism refers to the situation where two colour samples appear to match under one condition but not under another; the match is said to be conditional. Metamersim is usually discussed in terms of two illuminants (illuminant metamerism) whereby two samples may match under one illuminant but not under another. Other types of metamerism include geometrical metamerism and observer metamerism. Two samples that conditionally match are said to be a metameric pair. If two samples have identical reflectance spectra then they cannot be metameric - they are an unconditional match.