I have not had a problem...have used black pen on pure white fabric and it came out perfectly. Just love them! However, I am currently using them on a tan batik for an appliqué quilt, and after ironing there is a thin, faint white line where the ink was. It does not bother me because all the batiks and appliqués are so busy and the line is so faint, but I would not do it for a quilt going into competition. Give it a try on scrap fabric, and see what you think.