I have them on a couple shotguns, including my target O/U. Here's the deal....
I did about a year's worth of testing. First, I removed the bead from a target barrel on an 870 TB and shot it like that for awhile.
My scores at trap, wobble and Chinese Trap didn't change.
Then, I added a Hi Viz F/O sight, using the smallest I could find (3mm) in green.
My scores didn't drop, they may have improved. My hits were noticeable better, more solid.
Later, maybe a year ago, an old friend (77 at the time) was having a very bad day at the range.
We were shooting Chinese, a game where he shines usually. However, a stroke had impinged on his vision and he was having trouble hitting 10 out of 25. He's the only guy I know who had gone straight at Chinese so this was really bothering him.
I had an idea and lent him my O/U and asked him to try a couple with it. He smoked the first pair, dropped one out of the first ten.
I gave him a spare fiber, which he mounted on his Model 21 with scotch tape and tried a few more. More hits.
Now, he's shooting respectably with F/O sights on that 21 and various other fine guns he owns.
My theory on these is.....
THe F/O bead makes it easier to precisely position the barrel correctly in your peripheral vision, making consistency attainable.
As always, it's imperative to keep your focus on the target.
IMO, most F/O beads are a little big. Try the 3mm size well as the 5mm.
As for color, I do best with green, but try red and even yellow.
And,while they work for me, YMMV.
HTH....