Choosing the color and fabric for your welt cord is a personal choice. Go with self-fabric for a classic look or get creative with a contrast color, texture and/or pattern.
I like using the same or similar weight fabric as my slipcover fabric for making the welt cord bias strips. For the cord, I get good results with this washable, size 5/32 quality. This cord has a poly tissue-like core and wrapped with a braided cover. Find it at The Zipper Lady.