Sun Activated??? 

Photochromic is the correct term for sun -or- UV activated pigments.

In short Ultra-violet (UV) light changes the molecular structure of the photochromic inks and makes them absorb color. The inks undergo a temporary physical change in which the molecules change state and appear colored.

When the UV source (sunlight) is removed, the molecules return to their original state and the inks lose their color.