wolfkitty

Thread to match the color of the light fabric in the quilt. I can't tell if it is white or off-white, but whatever goes with that.

QuiltE

IMHO it would depend on whether this is hand quilting or machine quilting ... and what type of design you are doing.

I would audition a pale green. It would help to show the stitching some, yet not have an in-your-face look to it.