I use YLI 100 wt. silk thread and match the color to the piece that is being needle turned tack stitch appliqued. If I can't match exactly the piece I go with a darker shade as close as possible, because if you use a lighter shade it reflects the light and is more noticeable. If I don't have the right color in YLI 100 wt. silk, I use 60 wt - 2 ply (same as I use when piecing blocks for my miniature quilts.) Some greens, blues, beiges, browns in fabric colors can be appliqued with neutral threads in the taupes, beiges, greys (as appropriate to the fabric color)as they seem to be able to blend into the piece being appliqued.