Just wondering what color thread to use when making the bargello quilt with the stark colors.

Now you have me wondering. Maybe a neutral gray?

If you mean when sewing it together, I use the medium colr if it's monotone. For most quilt piecing, I use either light or dark gray, depending on the overall color of what I'm piecing. Sometimes, beige.

Ditto that!

I use gray or beige for piecing most of my quilt. I'll use a dark gray if the quilt is made of dark fabrics.

beleive it or not, I use light lavender for a lot of my piecing. It seems to blend in very well no matter what color I am using with the exception of black.