Grading in red is generally avoided, nowadays. Red is closely associated with failure/danger/bad, and feedback should generally be constructive to help students learn and grow.

I usually like to grade in a bright colour that students are unlikely to pick: purple, green, pink, orange, or maybe light blue (if most students are working in pencil). Brown is poo. Black and dark blue are too common. Yellow is illegible. Red is aggressive.

Anyway, I'm guessing they just graded everything in green. The only time I've ever graded in more than one colour was when I needed to subgrade different categories of grades, like thinking/communication/knowledge/application. In that case, choosing a consistent colour for each category makes it easier to score.