How to separate dialogue NOT spoken/within quotations?

It is sort of like inner dialogue but between multiple voices/people. Imagine someone with multiple personalities, and they all can communicate with each other in the mind. How do I make it obvious who is talking? In Google Docs I make each character in this situation have a different colour, but for a physical book or on wattpad how could I manage to show who is speaking without colours? I've thought about how they each have completely different personalities and traits, but that doesn't help if they make a basic comment that wouldn't show personality. Would I just use quotation marks? Or perhaps put the dialogue in the mind within apostrophes? Any suggestions, help, examples, etc. are welcome. If this isn't labelled correctly or something I'll fix/remove this. Thank you!