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.