I just want to say a big "thank you" for this post! I read it back in November and went out and bought the markers. I finally marked a quilt this week, using the blue marker on a red, black, and white quilt. I put it in the wash, adding a little water (mine is a "smart washer" and wanted to make the quilt feel heavier to get it to add more water). As I poured water over the quilt, the lines completely dissolved. After washing the quilt with a few color catchers, there is no trace of the markings.
After reading horror stories of other marking methods, I'm so happy this one worked so well.