After updating K9 on two devices, one running Android 6 the other running Android 12, I find that when I select an email to read on either device the selected square on the left turns bright red - the colour of danger!
it is helpful to have the square on the left turning to a colour after it has been selected, but that colour should be the colour which has been assigned to the account to which the selected email has been sent. So, for example at the moment my email account which has a green stripe down the side of it is being saddled with bright red, when a green square would be an appropriate match and would be helpful. An email in the account which I have assigned blue to should turn blue when I select it. And so forth.