I use a dark grey felt for most of my fretwork. The color varies a bit so it gives a sense of depth to the projects.
I also use wood backers that are stained darker then the top layer. Wood is really helpful when you need extra stability to the project.
I have used cardstock, if you use scrapbooking cardstock it should not fade as much as other types. I use cardstock when I can really tell exactly where objects start and stop. Letters, shapes, silhouette, some logos. When doing more complicated fretwork pieces some of the exact lines get blurred, it is hard to tell where the leaf ends and the flower petal begins. I also don't like having a really busy pattern behind the fretwork, I think it distracts from the wood.