I've done my own test with these pens and they worked fine. I think the key is whether you are going to wash the quilt or not. I wouldn't use them on a wall hanging or an art quilt.
I recently used a green one on a white on white whole cloth quilt. The lines disappeared with the iron as they should. I then washed it in the washing machine with Tide. I did not let it agitate, just soak. Then rinsed it by machine, folded it up and put in in the deep freezer. I came back 2 hours later to a frozen quilt - but not a mark on it. Unfolded the frozen quilt, tossed it into a medium heat dryer and it's beautiful
I agree, - always test a new marking product before using.
As for dark quilts, I haven't used one on that yet.