When I had a bunch of colors on a quilt, I had the same problem..........what color to use. Someone in the past had suggestsed using a beige, pale green or pale gray. I went with the gray and love it. My backing was a solid peach color too so it really showed. I had no white in this quilt but I think sometimes a completely off the wall color sometimes does the trick.

I only have three colors for sewing quilts. White ,cream and gray. Gray seems to be the answer for everything.

My first thought is do you want the quilting to show or do you want the piecing to show. Is it necessary to cross the colored blocks? Like everyone else a picture speaks a thousand word.

I am using Gutterman invisible thread on top and and a color that matches the backing for the bobbin. Have been having good luck with it.

Off white, or beige works for me on multi colored quilts.

The quilting stitches will be most obvious in the background, and for that reason I wouldn't stray too far from the background color. Stitching isn't as obvious on printed fabrics, so I consider the background and backing first when choosing a thread color.

There are so many variables, including how densely you plan to quilt, your skill level, the thickness of the thread you choose, the particular fabrics, the pieced design.

My best suggestion is to take some scraps and make an improv block. Then try out a variety of thread and see what looks best. you can then use this block to make a pillow or a storage bag for the quilt. Or if it's really bad, you can designate it for animal shelter use. But taking that time to do some trial runs will give you confidence not only in your planned FMQ moves, but also the thread, tension settings, batting performance, etc.

Meant to add -- I am not a fan of high contrast threads for FMQ -- with very few exceptions. I like to match or go lighter. A pearl gray is often my favorite choice, or a creamy off white.

RST has some good advice. If I'm doing very dense quilting, I like to use a matching color so that the texture is visible but the heaviness of the thread does not distract from the piecing (unless you are thread painting!). This might mean changing threads several times. If you have a great deal of white, especially if it links together so you could FMQ from one background area to another, I'd probably go with white there. Then depending on your colors, a tone (grayed) color would look good with gray or taupe or a sage green, more saturated colors will be trickier--pull out several colors and puddle some of each on the quilt top, overlapping the colors you want to quilt to see what looks best. I'm always surprised when something that I really didn't think would work, does! In a class with Jamie Wallen this summer, he said that he often used a tone lighter than the colors of the block so the quilting doesn't overwhelm the piecing.

I go with variegated also. Pick one that has at least a bit of blend, rather than clash, with your block colors. It will add interest to your plain areas and not detract from your blocks.