Unless you want to keep using the old version, literally going back to the old user interface is practically not possible. A lot of the libraries and APIs used in K-9 Mail 5.600 are outdated by now. We’ll certainly not throw everything away and start from where the app was 3 years ago.
However, if you can let us know what exactly it is that makes using the app difficult for you, we can work on that. Messages in the Unified Inbox and regular folders have been color-coded in 5.600. But the new version is using less of those color chips than before. So I’m not clear on where the problem is.
K-9 Mail 5.801 will display the account name (“Demo” in the screenshot) in the top bar where in earlier versions only the colored bars on messages would indicate which account was currently selected.
Please create a new topic and describe the areas of the current app that are hard to use for you as a color-blind person (and maybe what you think it was in the old version that made it easier to use).