The exact effects of UV enhancements are speculative as humans cannot experience what fish (or other animals) experience, but we know they have UV receptors (which we do not) among color receptors in their eyes.
Based on personal experiments, the effects of UV enhancement are magnified as the amount of visible light diminishes. The easiest way to see this is to take a UV enhanced bait (away from strong visible light) and expose it to UV light to make it "glow". Then, keeping the UV exposure constant, move the bait into visible light and you'll see the enhancement wane and actual colors become the prominent display.
Think of UV factors as a flashlight... in the dark, it has quite a noticeable effect, but at midday the effect is overpowered by the natural light.