Photochrome
You give us those nice bright colors
You give us the greens of chemistry
Makes you think all the world’s a funky lab, oh yeah!

Zap the molecule with UV and it turns green. This is due (as I understand it) to the molecule changing to another state (isomer) — and not simply fluorescence — where it then has a different absorption spectrum, so in this example it looks green. When you remove the UV, it reverts to the original state and becomes clear again, and it’s doing this quite rapidly.