Photochromics (PCs) change from clear, when indoors, to color when outdoors. Specifically, PCs change color in response to ultraviolet (UV) light, usually from the sun or a black light.
Photochromic behavior can be either Reversible or Irreversible. (Technically all photochromism is defined as reversible but, for simplicity, we will use the term irreversible photochromic to describe irreversible a photochemical reaction that yields a visible color change.)