I'd also like to see already chosen dialogs color marked too.
Maybe:
- White ----- Current Player ---- Unread
- Li. Grey -- Current Player ---- Partially read tree
- Dark Grey - Current Player ---- Fully read tree
- Red ------- Player One
- Blue ------ Player Two
This is assuming you never get to revisit duologs, else you'd need a light red (pink) and light blue (bink?) as well.