The first 2 chevron quilts I did I quilted with a white thread. I wanted to keep my plain sections (the white) white..I didn't want to interrupt their "whiteness" because they were there to provide contrast and "rest" so the coloured sections with all their different shades and patterns would shine. The white thread quilted onto the coloured chevrons looked lovely...a nice add of contrast that didn't fight with or get lost in amongst the coloured, patterned fabric. And then the white thread on the white fabric just looked serene and crisp. Now...the third quilt, the pink quilt, I did the exact opposite, using a pink thread..I'm not sure why, it was so long ago but right from the time I started quilting it I had my concerns. Now, 4 years later I still don't like it as much as the other quilts. The pink thread on the white makes the whole quilt look far too busy for my liking.
On another quilt I just finished I had a huge area of just dark brown....the patchwork and colour was in the central motif. I chose a neutral colour thread for the central motif so that it didn't interfere with the motif at all, and then used a dark red thread on my dark brown background....the red blended well with the brown but still gave a peek of "hello" and actually helped bring the brown fabric to life as it was such a large area of nothing it needed something to please the eye.
So think about what you want to achieve, what parts you want to highlight with contrast, and what parts you want to highlight with "quiet". It's your quilt, your preferences. And once you've decided, spend a few days auditioning the threads....unravel them and lay them on the quilt, and lay some other "what the heck why not" colours as well. Leave them for a day or two...look at them in different lights, and you'll find the ones that you want.