Well, the emerger is going to be a pupa rather than a larva, so you would want to imitate something that looks like this specimen
There's one important feature you can't see in my pictures: just when it's about to emerge, the fly supposedly secretes some air bubbles underneath the outer skin of the pupa to buoy it to the surface. I haven't seen this myself, but I trust LaFontaine's observations. The air bubbles give it a sparkly look, which he started to imitate with the antron bubble.
His rationale was that the structure of the yarn traps little air bubbles in the same way as the actual pupa, so the sparkling would come from the bubbles rather than the yarn itself. This strikes me as a little strange, since I can't get the patterns to trap much air and the sparkling in the yarn itself seems much more prominent.
Since the yarn is really meant to help imitate bubbles trapped under the skin of the body, I think it should be roughly the same color as the body. Trial and error might prove me wrong, but there's no apparent imitative reason to make the colors different.
I would tie the imitation with a brown/tan thorax and wing pads, and a green body and green antron sheath. I haven't fished a really good Rhyacophila
emergence so I don't know how well this would work, but those would be my choices based on looking at the insect.