I tie a generic dubbed nymph with some turkey pulled over the abdomen like a shell back. I tie in a second piece of turkey about twice as wide for the wing case.
I also like to tie them in generic hare and partridge soft hackles, both with and without the beads and contrasting back.
I am not sure if the contrast matters much on the nymphs, as far as fishability, but I agree with vc that it's significant to the human eye.
The reason why I am skeptical that it matters is two-fold. I haven't had any more success on the two-tone nymphs, and also, using flies that look different from different angles highlights the fact that the fly is drifting unnaturally. My theory is that the naturals don't tumble nearly as much as the imitation, so using a solid color imitates a bug that's drifting with some semblance of control.
Credit to the al's trico guy and his postulations on the now defunct llfs site for that.