Over the last couple years I have been retying patterns with UV and flourescent colors and have seen a huge difference. In September we had 5 people fishing the same fly and getting multiple hits per cast on Dolly Varden for hours on end. Similar flies without UV dubbing did nothing... to the extreme end of nothing.

Many colors I have used over the years and thought were great had started to fail, others picking up... Until I shined a UV light in my fly box and realized the materials I have always relied upon were glowing under blacklight I thought the fish were changing.

For example, my favorite huge rainbow pattern for years in the Kenai River was a yellow marabou muddler. Then after many years it failed to produce and I moved on, but always had the yellow muddlers to try on slow days. The blacklight showed my initial flies were tied with flourescent yellow marabou, but my new supply was not. My yellow marabou muddlers now fish like they used to...

How much difference it makes for mayfly patterns and such is obviously a very different question...