When highlighting passages in your textbooks, print-outs and study notes, it does help to have a consistent colour system.

You obviously have to make it relevant to your field of study. For example, lawyers are apparently taught to colour-code  using red highlighters for the holdings of a case, green for general law, yellow for facts etc.

A medical student might colour-code Latin terms, diagnoses, procedures etc.

This is my system:

The trick is not to go overboard with the colours. Only highlight what is necessary, because a completely colourful text will end up confusing you.