What kind off emerger, Bruce? If you are imitating the fresh dun abdomen, I'd go with very pale olive, gray or tan depending on the critter. I mean very pale, if you don't have these colors, white is best. For the nymph abdomen, the same color selection in the more common shades will work, provided they are lighter than the body. I rarely use ribbing darker than the body.
Well, that's they way I often fish my Ripe Nymph. Sounds like you are imitating the nymph. Baetids will split their wingccases subsurface exposing some thoracic color, but they only eclode (emerge) at the surface as far as I know. Anyway, which is the most dominate color? If olive, I'd go with the gray thread on an olivaceous body.
Based on what you've described, I'd go with gray - if only because olive thread often looks dark on dubbing blends with a lot of muskrat in it.
I noticed they have a very segmented...)
It strikes me as so cool that you're on the left coast and I'm on the right yet we can carry on a conversation?
Sometimes on a small dry like a BWO I will use the flat, 70 denier UTC thread, and lay a flat abdomen back and forth ending back,and then twist the flat thread, and rib forward. It produces a subtly ribbing effect that I want on the small fly. Then a bumped up thorax.