Precisely because I hated the sight of red ink on my essays when I was at school, as a teacher I invariably graded student papers in blue, navy blue, or black ink. And, indeed, for that matter, I don't recall anyone trying to alter a grade, but experiences differ.
I know some professors who use green for exactly that reason-it's much less "jarring" but still distinct from the student's work(when you're working through 200+ exams that need to be finished in a few hours, being able to recognize marks at a glance is important). A smaller number still use purple.
As for changing grades-unfortunately I've seen-and been able to prove-too much to not
always have the possibility of cheating in my mind.
One of the most blatant cases happened when I was a teaching assistant. A student scored a
very low grade on an exam-I think 30 or 40%. She took a friends's exam(who had a grade in the 80s), erased the name on it, wrote her name on it, and then claimed that the grade had been incorrectly recorded. The senior instructor thought something looked "fishy" and showed the exam to me. We both could see the erased name well enough to read it. We then went back to the records, found the name that we thought had originally been on the exam, and it had a grade that matched the exam paper in front of us.
As a consequence of that stunt, both students failed the class and had letters sent to the deans of arts and sciences(our college) and the college of nursing(both were pre-nursing). Also, that particular instruction now keeps copies of
all exams-if a student reports a scoring problem or a mis-graded problem, she won't even take the student's exam paper back-she just regrades from her file copy.
I've see students attempt to change numbers on exam, and enough other under-handed tricks to make your head spin. I use fountain pens for a variety of reasons, but one of the things I like is the freedom in ink choices. As I said, something written in Pelikan Red is as good as my having signed my name to it in the department.
As most of the students I deal with are pre-health(pre-med, pre-dental, pre-nursing) I consider it something of a civic duty to catch as many cheaters in the act as I can. Someone with no reservations about cheating on a class at the undergrad level is someone who I
don't want treating me.