Hi Folks,
Good question!
My understanding is that flashy flies are best for bright days on loughs with a sandy bottom that reflects a lot of sunlight. This causes the 'mirror' of the air/water interface to become very reflective of the lough bottom (and therefore 'cluttered' to the trout's perception from below) so you need something flashy to stand out against all that background noise. Quite how much flash you need I do not know! Or when it is too much, but get seen, get eaten? And a lot of the wee water beetles certainly give off a lot of flash from their bubble of air. So maybe the more flash the better on a bright day on a lough with a light coloured bottom (like those on the machair).
Also noticed too, in the very clear lough waters of Greenland, that when spinning for char, bright lures brought them in as a shoal and advertised the presence of something going on. So after maybe a dozen casts of nothing I all of a sudden started getting takes with a visible fish or two following on every retrieve. Move along the shore . . . . nothing . . . . and then after a dozen casts, the same again. I suppose this lends some weight to the idea of an attractor on the bob with something more realistic towards the tip, so that if the trout decides the bob is too flashy and gets suspicious, you have a more acceptable alternative for it to consider as it turns away from the flashy bob.
Anyway, back to my hard day's procrastination/work!
Andy