I only use Bottom Line thread for applique.
It's so thin it doesn't matter what colour you use.
The only concession I make is I usually use a light colour BL on light fabric and dark on dark.
I appliqued circles onto 8 quilts using 1 spool of light green on background squares of dark green, yellow, navy blue, ecru, etc. No stitches showed.
http://www.superiorthreads.com