A few years ago it was all the rage to get teachers to convert to using purple as a correction colour, because red was seen to be as too harsh and intimidating. Purple incorporates the calmness of blue, and it was thought it would offset that knee-jerk "Oh my gosh, my paper's covered in red!" feeling.
I haven't read the article linked yet, but I suspect we'll find this effect is seen because of experiences with the red pen, and not that red is an inherently demotivating colour. In fact, I bet if they conducted experiments using teachers as the subject, we might find that red energizes them or makes them more critical/sharp/astute for the exact same reason. Red still implies criticism, just giving it, not taking it, to teachers.
