Hi,
In the side menu when you click on "Show accounts" you see circles on the left with the two-letter abbreviation of the domain for the accounts. I think, using the first two letters of the domain can cause a lot of confusion for some users if the user has more than one email account for that domain even if their background color is different. For example, I'm seeing OU(Outlook) for 5 different accounts or GM(Gmail) for 3 different accounts.
I think either allowing the user to customize these two letters or adding a toggle button to use the first two letters of the account name* instead of the domain can also solve the problem. This way, the user can use a color for the domain and use the letters to differentiate which email account is in that domain.
For example, the user can use blue color for Outlook, and red color for Gmail and use two letters to differentiate which account it is in that domain.
[*] the account name refers to the setting in Settings > Accounts > {your.email@example.com} > General Settings > Account Name. Since it is exactly the same as the user's email address by default anyone who wants to use the first two letters of the first half of the email address can use it by enabling the option I mentioned above, If anyone wants to use a custom two letter can do this by changing the account name.