This is the wrong forum for this as the example is about dries/emergers but here goes as I believe it may have relevance to how we fish spiders/wets/softwings.
When it comes to dries to match whats hatching and been taken (Olives) I "currently
" believe IN GENERAL that after stealth (you, your flyline, your leader,your casting etc) , presentation (cast, angle of cast, drift of fly) that the next important bits are size & profile followed tentatively last by colour - though I know many anglers who strongly believe that colour in dries can be more critical more times than we might believe - i really do not currently have an opinion on this matter except to say I would err on matching colour but currently do not panic about it.
Up to 3 or 4 seasons ago for olive hatches I fished what were deemed by history and its experts to be dun imitations with very arbitrary success. Off course the reason was simple, they were taking emergers or bulging at nymphs just subsurface and the DUN imitation which like most so called DUN imitations was a poor Dun imitation and an even poorer emerger imitation. A couple of seasons fishing emergers both curved to cut thru film, and straight hook versions to sit on/in the meniscus it became very apparent that sometimes during a hatch that some trout did indeed show a preference for one or the other, and that you may need to change the profile of your fly during a hatch to take full advantage - assuming of course you are hell bent on fishing dries.
What this has to do with spiders and wets I am not sure except that during a hatch I suspect that if my top dropper were to delay a little longer in the meniscus it may be advantageous. Since olives are the bread and butter on my local I intend to tie a few of Mikes softwings with a little more hare dubbing by incorporating a few extra guard hairs at the butt of the wing and see if they can perform to my theorising. Off course I could put on a full dry emerger on the dropper but that would only confirm what I already know to work resonably well.
Thinking too much can do this to you