While I hardly think a light underbelly is necessary as most insects are lighter underneath, the absolute first time I ever caught a trout on a nymph, it was on a woven body stonefly nymph I purchased at a local fly shop because it was dark on top & light underneath.
If I remember correctly, that nymph had a hackle tail and a tiny tuft of grey hair for a wing case. While it isn't the same fly, PAFF's very own Jerry Coviello demonstrates the weaving process here:
While woven bodies nymphs tend to be wide, by using thinner yarn or even string you can slim down the profile.