Get the one that holds what you want to carry routinely and don’t unpack often. You may find you will need more than one if one class have a substantial!y different size or configuration. Don’t get one to carry textbooks because they are expensive and persona!ized.
Look around campus and note how many bookpacks are unattended and look like many others. Don’t put anything in it you don’t want to lose or routinely haven’t with you. Much like you do for checked baggage, do something that’d makes them recognizable to you.
My first reaction was a blue one. Truly, however, I think a bright color would make it easier to find.