Back when I was in the first grade (circa 1973) my teacher surprised us by grading her papers with a green ballpoint pen. I remember it was a standard-looking Bic with a clear plastic barrel.
Anyway, I asked her about it, and she told me it was because green was not readily available--only teachers could buy them back then (for the young ones here, pens used to be black, blue or red. You'd be hard-pressed to find any ballpoint in any other color--at least in my neck of the woods).
So it wasn't about protecting my feelings. It was about preventing forgery.
General Tsao