(Think fish vision here...) Ultraviolet rays are the only "light" source that penetrates through deeper water. In order for a fish to see the lure in "natural" color (i.e., red), the lure itself must be conducive to emitting a ultraviolet source back before the end of the color's wave length. If not, all colors turn gray at the end of their wave length.

I only know of one tackle manufacture that has taken UV serious for salmon fishermen- Radiant (Canada).

As far as a FF for blackmouth, I have had mixed results. I have used the glo version in front a spoon and just don't have enough confidence in the deeper water. When fishing shallow, (<70') the FF really produces well. Especially during the sand lance spawn. I'll run a Coho Killer about 30" behind the FF.