In the uk some streams are gin clear. It is good practice therefore to cast to fish you see rising or taking nymphs just under the surface.

It needs a lot of stealth because if you appear in the fish's circle of view they will just dissapear. So getting behind the fish and upstream casting is the order of the day. Because you are casting over their heads and allowing the fly to drift down, any splash, or drag of the fly will result in a blank day. Also the fly needs to be as exact representation of the original as possible both in size and the way it drifts into the fishes taking zone. However I doubt that the number of legs or the exact shade matters too much. I think its very much behaviour and presentation that is paramount.

If the fish is taking flies from the surface, then fish a surface fly, if taking emergers, then just below, or if boiling for nymphs then get the fly down. They certainly will not be checking for thread shades or numbers of tails.