Doing a pantograph and the backing is red flowers on black background.

I vote gray. That way you will have similar contrast through the quilt. White black or red will give some areas of high contrast and other areas of no contrast at all

I made a similar colored quilt last year. My quilter used grey. It let the fabrics shine and the quilting is subdued.

I am going out on a limb here...I would use a yellow. Not scream in your face glow-in-the-dark yellow, but a nice creamy yellow. It will show up on everything with out taking the limelight. I choose colors that dont disappear on some of the fabrics and stand out on others. JMHO I might consider a tan also...

Lemonaid (Permanent Core) is a great blending thread. Does not look yellow anywhere, not even on the white. I would not use the gray, verigated or black. They are too stark.

Lemonaid (Permanent Core) is a great blending thread. Does not look yellow anywhere, not even on the white. I would not use the gray, verigated or black. They are too stark.

I love the varigrated red idea.

If you use white it won't be visible on the white. If you use black it might scream back at you in the white and red areas. I would try gray and cream and see which one I like better. Just lay the thread over all color choices and go with your gut.

I use gray and silver a lot when quilting.