So let's play a game; think about what kind of work you do doing your current job or current course. What colour uniform would that job entail in the 24th century? I'm studying city planning so I guess I would be in sciences blue. My undergrad course was in Modern Languages so I think that would also be sciences blue, to go by Hoshi Sato. Uhura's job seemed more technical so I imagine that's why she was in services colours.