You most often see color/shade contrast in fly tying in the round on the pattern….a contrast of materials wrapped up the shank and by virtue of a contrasting ribbing of some sort the fly appears segmented and/or of different colors.
Another technique is to over lay the body (abdomen and thorax) with a darker material. This is seen in the Skip (Morris) Nymph, the Czech Nymphs and in this instance (my pic) a bead head pupa pattern. I didn’t tie the fly (not sure how I came by the few I found in a box’s compartment) but I noticed the backstrap and found the material (dark biot) interesting, if too sparse.
You get the idea of the overlay of material creating a contrast (darker on top/lighter on bottom). Ribbing for the suggestion of segmentation is usually tied in at the same time as the backstrap material. The ribbing binds the material down atop the body material. This darker over light idea is frequently used for the wingcase over the thorax. Pic of Callebaetis Nymphs, Upper Left