We have had this discussion recently and I thought I'd up date you to as to an experience I had recently. Until now all experiences have been favorable.
I used a purple Frixxion to mark where the center was in my embroidery design space. After stitching I ironed the purple marks and they turned WHITE!
Here's a pic of what they look like. Hopefully they will wash out when Erin washes the apron for the first time!