My concern is that, if the marks still reappear (however faintly), it probably means that there is some remaining chemical left in the fibers which may or may not ever fully wash out, and these chemicals may cause the fabric to deteriorate faster over time. The chemicals in some of the old dye colors caused fabric to deteriorate, which is why quilts from a certain age will have shreds or holes where a certain color of fabric was used. I'm not enough of a chemist to know which chemicals might cause deterioration over time.