I also hand applique, and I too use silk thread, but I use Taupe, it seems to 'hide' very well in just about any color or pattern you are using.
I agree with MTS that your LQS (or if you don't have one close by, it will give you an excuse to stop along the way at one! ) tehehehe
anyway they should carry silk thread.
Enjoy your trip and your appliquing.