To have your messages colorized on an account-by-account basis, you can achieve this by creating Filters to assign Topics to incoming mail, then colorizing the Topics.
- Open Preferences (macOS) or Options (Windows) > Display > Topics, and create a new Topic for each account you wish to colorize. Make sure that the "Apply topic color..." checkbox is selected and that you have a unique color per Topic.
- Select Message Filters... from the Tools menu.
- Select the account you wish to work on.
- Set the criteria to "TO > IS > YOUR EMAIL ADDRESS."
- Set the action to Add Topic, then select the appropriate Topic for that account.
- Repeat steps 2 through 5 for each account you wish to colorize.
This technique will for new messages, but what about messages already in the folder? Easy! Perform a select-all command to select all of the messages in your inbox, then type the "t" key and start typing the Topic you created for the currently selected account. Auto-complete against the topic name, and all of the messages will then be assigned that topic.