I also use the pens and love how easy it is to iron off the marks....I have seen the lines left on darker fabrics, but don't think it is bleaching the fabric as it does wash out....always test fabrics with the pens to make sure there are no issues....I found them at the LQS in lots of colors, but would love for them to make one in white or at least something that shows on black/dk. brown fabrics.....am quite sure they never imagined their pens being used to mark quilts!