I ran my own test a couple of years ago. I took about 15 different fabrics and tested the frixion pen on them. I scribbled on them, ironed them them threw them in the washer and dryer. Then I placed them in the freezer. the lines came back faintly on some of them. I re-washed them and put them in the freezer none of the marks came back. I have had the the pen leave a faint line that almost looks like it is lightly bleached after ironing on some darker blues. So far it has always washed out for me.
I have played a little bit with the frixion highlighters. So far I love them for marking multicolor quilts. The will show up on dark and light colors. So far I have had good success. I always test any makking pens I use on scraps before I use it all over a quilt.
Years ago I heavily marked a wall hanging with one of those blue markers that come out with water and had a terrible time getting it out. Lesson learned! Test on scraps first.