When work is marked, it usually only includes a single grade with a sentence of constructive criticism. However, the benefits of using categorised marking ensures that the student is totally clear on where his/her strengths are, where they need to improve, where they lack understanding and any parts of the exam criteria that they have missed.
Using different colours or symbols will also ensure that students take more notice of specific comments as they are given a thorough analysis and a clear and plausible route for improvement. Who knew just one piece of work could hold so much insight?
Ah, the dreaded red pen. Students, parents and SLT often cringe at the sight of the red ink used by a teacher. Maybe because it's a symbol for blood?
I think it's more likely to be linked to the idea that teachers from a previous age would use just a red pen to mark work and would be likely to mark negatively; pointing out mistakes or circling poor work.
However, when a teacher marks their books now, they are usually (or hopefully) better trained into what marking works and what will benefit their students' learning.
And I do use a red pen. Alongside green, orange, light blue, brown and even a pink. These colours signify different bits of advice for my students which they can understand with a quick glance. This idea was stolen largely from Joe Kirby, who suggested using symbols for a quick marking technique which the students would understand. For example:
? Analyse Miller’s intention in your point
+ Choose a more relevant quotation that shows their relationship
* Improve your link to the audience’s interpretations