I would NOT use a Frixion marker. People go back and forth about the marks reappearing in cold temps, and then someone says I don't plan on keeping my quilt in the freezer - the point is, the ink stays in the fabric even after washing. Additionally, the manufacturer says they are not designed for use on fabrics.
I used slivers of white bar soap on a black quilt. It's harmless and will easily wash out.