Florescent will glow under a black light. These colors are quite bright. You add them to the oils, before you mix in the beads. You can however dust them after they are completed and dry also. (Be cautious when purchasing some powders that say Neon. We stopped using them and switched to a different brand because they had an awful fish smell to them that was not covered by the fragrance.)
The powders can leave a slight residue in the containers. The powders mixed in before baking will bake into the beads.
The powders come with a small measuring spoon. We suggest you start off with the measuring spoon. If you use too much it can weaken the plastic when it gets too hot in the car. With the Fall and Winter coming, there should not be too big of an issue.
You can also use mica powders for coloring or sparkles. Remember, too much of anything is not good. We do sell a LOT of powders and mica powder.