My view on UV vision is based on practicality. Let us assume that adult trout do have an element of UV vision. The question then becomes, since we humans do not see into the UV, how can we imitate the "UV colors" that trout supposedly can see.
The problem is that we imitate the size, shape, COLOR and behavior of the naturals we see. When we seine or capture a natural caddis or mayfly adult or aquatic nymph or larva, we imitate the colors we SEE. Since we don't SEE the UV, we CANNOT match the UV "color" of the natural. In fact we have THREE problems.
The first is that we don't know the UV "reflectance" which is the UV color of the natural. The second is that we don't know the UV "reflectance" or UV color of the fly tying materials we use to imitate the natural. Thirdly, we cannot know what blended color of the visible natural colors and the invisible (to us) UV color looks like to a fish which can see both visible and invisible (to us) UV colors.
We pick colors of the fly tying material to match the colors we can see. There is little else the average fly fisher or fly tier can do. Can you imagine walking into Slide Inn on the Madison River and asking Kelly Galloup if his flies match the UV color of the naturals in/on the Madison?