Pairs of (chemically different) colours that can barely be distinguished in one type of illumination, but show a noticeable colour contrast in another type of light or when viewed through a filter, usually a red filter. Four-colour reproduction systems (offset printing, colour copiers, inkjet printers etc.) cannot copy metameric effects. Either the metameric colours will lead to identical four-colour reproductions (with a complete loss of the metameric effect), or the metameric colours (which appear similar under normal light) are reproduced in two different colours, which makes the reproduction obvious.
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