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Two pieces of card, or cloth, or infact any material at all are metamers or form a metameric pair when they share the same colour but the light they reflect is different.

The effect is noticeable when the the two colours are worn in the same suit and the person walks into a different room/building with altered lighting, as the pair suddenly look different from each other.

So what, you might say?

Well those of you with a physics background would realise that this is a big problem for those who would like to identify colour of surfaces with the the wavelength of reflected light. This, btw is the standard approach to colour that is taught to school children all over the globe, and most people never stop believing it.

How can two colours be the same when the light that they reflect is different? That's the million dollar question, solve that, and every fashion house in Paris will want you tomorrow.