Bobbin thread choice
If I use Aurifil 28wt, for the needle thread, what should I use for the bobbin head?
I would use a matching color in the Aurifil 50 wt—on the orange cone.
You'll probably need to make some tension adjustments because of the different size of the two threads, but they should be pretty simple. A couple of practice runs on a scrap quilt sandwich and you're ready to quilt.
You may find a 90/14 needle works best, particularly a Topstitching needle, due to the diameter of the 30wt thread.
Using the same color in the bobbin means that if your tension is a bit off as you quilt some of the motifs, there will be no pesky contrasting thread blips on the front side of your quilt.
Thank you for your question.