I always use bobbin thread in the bobbin for applique to reduce bulk, and the best thread on top is that which is closest in colour to the fabric.

I was wondering that too. Use the silk thread in the bobbin also? Do I need to adjust the tension to use silk thread?

I've done appliqué before and changed the thread to match but the project I'm working on now has toooooo many colors and small pieces to keep changing thread. I didn't want use black. I'm using a medium gray with just a small zigzag stitch but was lookin for ideas for more neutral thread for next time. I've been looking at Aurifil and Superior online but it's difficult to tell the exact colors. I need to find a shop nearby. Or I was thinking of coloring the thread with fabric coloring pencils when the appliqué is finished. That might be cute. Thanks for all the good responses posted.