I love my Frixion pens. I use them often to mark myFMQ lines. So far they have worked well. I have not used them much on darks. Yesterday I was machine embroidering a shirt for my dad. I marked a box on a medium blue t-shirt to keep my embroidery straight. I used a black Frixion pen. When I ironed it out it left a light line. I had an OMG I ruined it moment. I washed the shirt and the line is gone. I will have to make sure I test it on my fabrics in the future before making a whole quilt