Glow powders absorb light energy so a clear medium must be used because this lets light in for the pigment particles to absorb light.
When in darkness the pigments discharge (Glow) this is sometimes referred to as an after glow effect.
The larger the Glow Powder particle size the better the glow effect. having large partials of powder in paint can make them hard to work with but its amazing what projects people have made using glow powders.
Tip: Its important to think about the lighting conditions of an area before starting a project.