I was bored the other day & went shopping (it always works). I picked up some embroidery floss, sewing thread, & a ball of baby yarn.
Here is a new sample on the orange warp. For the fine background weft I chose something really fine: lilac sewing thread; and for the brocade weft, I used the baby knitting yarn in mint green.
Interestingly, the lilac thread reads as the same tonal value as the orange warp, and practically disappears in the cloth. The green, on the other hand, is a very much lighter tone than the warp, and the contrast is good. If you photograph these yarns in black & white, the tonal similarities and contrasts are more evident.
The reverse side is interesting. If you zoom in, you can just catch the little lavender ground wefts in the areas where the brocade wefts float on the face.
Conclusion: this is a good choice of weft yarns for this technique. You will recall that the warp is 8/4 cotton used double at 12 epi.