Last time I checked the sun isn't a color...
Why do fish use a form of cover on sunny days? I'm guessing because they can see it.
Well sunlight is a colour, a spectrum of colours; some we can see and some we can't.
Same with fish, they see a different spectrum of colours then us.
Colours also look different in different weather conditions and different depts. It's amazing.
Fish use cover on bright sunny days only if the water is clear. If the water visibility is low, sun don't matter.
This is because they can see moving objects on the riverbank and in the sky which spook them.
Also because they don't like sunlight.
Plus also to hide from predators in the water, they go to deep dark spots.
There is also a reason why fish's belly is bright silver and chrome and why their backs are blue, green and black.
If you look at the fish from below it's harder to see silver bellys looking up at the surface.
What you are suggesting makes sense for us, but it's hard to say how exactly it looks to the fish.
Some people think that the colour of yarn doesn't matter, but I beg to differ. Colour is most important in my opinion. Shape and movement are not as important.
If I ever meet you on the river, I'll recommend a book for you to read if you are curious.