I have discovered that if you use the dark pens on medium/dark fabric, they bleach the fabric. I marked a border on a red fabric, needed to adjust my markings so tried to iron out my first marks. I had permanent white lines. Since that experience, I have done lots of experimenting with the same results. Our little quilting group has tried everything to remove the lines, and they don't come out. I will continue to use these pens on light fabrics, but I've learned my lesson on darker fabrics.