Ah, got cha! The colour of fluorescein depends on the charge of the
material it is bound to. Chemist actually make use of the effect when
they titrate chloride with silver nitrate solutions. Fluorescein is added
as an indicator, which absorbs to the silver chloride precipitate. If
cloride is in excess, the particles are negatively charged, if silver is
in excess, they are positive. So you get a sharp transition from green to
pink and back.