Metamerism is the phenomenon where colours will match under some lighting conditions but mismatch under others.

Metamerism can be a serious problem for manufacturers who combine different materials or materials from different sources into one product.

For example, a customer expects all the similarly coloured parts of a car to match if it is a sunny or cloudy day. Someone who purchases a coordinated outfit expects the jacket and trousers to match in daylight as well as under the fluorescent lights of the shop.

Metamerism index is a single-number index which indicates how well two materials that match under one illuminant will match under another illuminant. Typically, CIE Illuminant D65 is the reference illuminant and the test illuminant is one chosen depending on the end use of the material.

For this index to be meaningful, the materials should be a good match under the reference illuminant.