What is "leuco dye"

alt. Any dye that has a leuco form. n. Any dye that has a leuco form.

A leuco dye (from the Greek 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 transformation 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.