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A leuco dye (from the Greek λευκός leukos: white ) is a dye which can switch between two chemical forms, one of which is colorless. Reversible transformations can be caused by heat, light or pH, resulting in examples of thermochromism, photochromism and halochromism respectively. Irreversible transformations typically involve reduction or oxidation. The colorless form is sometimes referred to as the leuco form.
Leuco dyes form the basis of thermal printer papers and certain pH indicators.