A teacher in the Boston area claims that the current procedure of marking papers in red is scarey. She uses purple because it is less scary. I think that it is just more from the politically correct crowd. They are afaid that someones feelings might get hurt.
http://www.boston.com/news/education/k_12/articles/2004/08/23/harshness_of_red_marks_has_students_seeing_purple/
http://www.boston.com/news/education/k_12/articles/2004/08/23/harshness_of_red_marks_has_students_seeing_purple/