As there is white in this quilt, I would most likely go with a light grey or light tan based on what colors you actually have in the scraps. I do like Barb does, pick some threads I think might work, then unwind and place it on top of the quilt making sure I have covered each type of fabric I have in the quilt. There is times, I also find a light yellow works fine when I have white. As there is a lot of white, I would not use a dark color unless I really wanted to make the thread a focal point, because a dark or bright color on the white parts would do that.
OH, I might actually use a white thread depending on how much white and what the other colors are.