For scrappy piecing - when I have dark fabrics - I try to use up all the small amounts of thread that are on the bobbins and spools. Have to change the bobbin fairly often, but then I get empty bobbins for my 'real' projects.
My 'go-to' color is that smokey grayish green - when I can find it - or a medium sagey green. It is amazing how that color just sort of disappears on most fabrics.