if you are planning to do an all over edge to edge type quilting i would use a neutral- like light gray if the fabrics are mostly light- a medium or dark gray if the fabrics are darker. creams work well too but grays tend to blend in & disappear. i don't know many people who change threads often when doing free motion quilting but it is your choice. often i will go with the background- when i have alot of colors in the blocks.