I think a limey green would blend in well. Give it a try.

have you considered using invisible thread?

My "go to" colors when I can't decide have been a pearl grey, pale lime green, pale yellow or off white which I rarely use. Mostly use the lime green as I think it would work for your quilt. I used it on a red, purple with neutral background quilt once and was amazed how nice it came out. The pearl grey or pale grey works great too.

I may be the odd one out, but since I've only done stitch in the ditch or topstitching along the seam lines, I would start with the stars using white thread, then use green thread for all the green areas, and purple or lavender on the purple areas. I usually use 505 spray to stabilize the three layers, and then start in the middle as much as possible smoothing and repinning as I go. I like the piecing to be the focus, not the quilting. However i have seen some beautiful quilting but it is beyond my skill level.

I like the idea of light gray or lavender maybe a variegated. I have a variegated gray I like to use. Take your quilt to the store and audition different colors by laying the thread out on top of the colors.

Yes, yes, yes!

If I had a choice I would choose a pop of color. But I'm one of those quilters that machine quilts over applique. LOL

Wow, your quilt is awesome! My LA almost always uses a varigated thread that really makes my quilts beautiful, she has so many different colors to choose from and I'm sure there is one out there with purple, lavender, and greens. Your colors are beautiful together.

Stunning quilt--I would probably use either Superior SoFine with a lavender/gray that is a 50# or the SoFine that is a very light yellow/green.