Science (incl math) is green. Blue is geography. Orange is sports. Pink is Entertainment. Art and Literature apparently can be either purple or brown. You're still missing yellow for history.
But once you reach the point where they're separate subjects, biology is green, physics is purple, chemistry is green if you don't take biology otherwise its pink
Whatever your opinions on these though, I present:
The one I was always certain about is that biology should be green. Chlorophyll and the photosynthesis based on it are just so important for nearly all life forms that green is very much deserved by biology only.
Math is red, Language is yellow, History is orange, Science is blue OR green depending on the focus (physical science is blue, biology is green), Music is purple, though I was a band kid so I usually used the bigger music folders you got from the instrument shop. Art is whatever color science isn't because I'm not going to use the folder anyway and the sketchbook is always either a painting or brown.
I could also hear arguments for switching math and language.
Green = bio / blue = my language / red = math / black = french / black = english / yellow = german / orange = geography / purple = history / other stuff a folder with flowers / stars on it.
Those colors I used in my last year off high school.
For me, green was chemistry, red was biology, purple was physics, and blue was math. Orange would probably be Spanish, and pink or any other colors would be left over for English, social studies, etc.
what foolish regionalism are you universalizing now
Apparently the answer's green, I mean I would have gone with blue (star trek science officers, for one thing) but whatever, why would it matter except in an unnecessarily strict conformist-centric education system... is this American bullshit I'm smelling?
edit: ooh this is accumulating downvotes fast, I've hit a nerve i see, let me help..