The term metamerism is understood as the effect that two things may look the same under one light, but completely different under another light.
In practical terms can be mooring at an example: If a yellow sheet of paper and a sheet of white paper under yellowish light are considered, both appear the same. If the same sheets are viewed under white light, one sheet is white, whereas the other sheet is yellow, so a completely different colour.
This metamerism can especially occur very strong, if different colours and methods are used for the production. The gray of a proof is often made of several inks of the 11-colour proofing systems, while a neutral gray in an offset print ist often made from only one colour: black. The risk of colour shift is therefore higher for the proof than for the pure black in offset printing. Even in colourpatches of fabrics and prints strong metamerism may occur.
Lighting can also be a reason for metamerism. A perfect proof-to-print-match unter D50 standardized light might well be, but under sunlight with UV influence or incandescent light in the evening there can be huge colour deviations because of metamerism.