i use silk thread for applique- comes in lots of colors= but light or dark gray, taupe or cream blends in well.
it is very smooth- fine- strong thread that disappears nicely- and makes tiny stitches easy.
yli is a good source for silk threads
and available in 50wt and 100 wt. i tend to use 100wt for applique- 50 wt for sewing when i want it to show-like quilting.