A glow paint pen is a pen containing photo luminescent pigment, which causes writings or drawings made with the pen glow in the dark. To glow in the dark, the pigment needs to be exposed to light first.
Most common photo luminescent pigments consist of either zinc sulfide or strontium aluminate, and a dopant. The dopant acts as an activator, changing the crystalline structure of the main substance, which causes it to glow in dark. By choosing different dopants, you can make glowing paint of different colors and varying duration of glow. For example, copper-doped zinc sulfide gives green long-time glow. Strontium aluminate pigments are more expensive than zinc sulfide pigments, but their glow is brighter and lasts much longer.