I think the corner colours were fine. In the first version of the app I used, only about 4 colours were available. Making more available would be good, but I think colouring the messages or even the account titles has mixed advantages and disadvantages.
Corner colour:
can colour account names, folders and message headers with even strong colours, with no readability problems - plus point.
Needs at least some space. Minus point on tiny screens. Only a 'tiny' minus.
If too small, can be hard to distinguish colour on small screen. Change your spectacles!
Subtle shade differences less visible.
? Neutral - choose different colours.
Background colour:
Dark colours make readability difficult in some cases. Even swapping foreground to white is problematic for some colours. Minus point.
Account names or folders in background, message headers by corner colour. Still has readability problem, but does not extend to message header or beyond. Plus/minus.
Ugly. Minus point. (For a good appearance, use colour and fonts sparingly).
Giving the choice (corner or background or both). Complexity - minus point. I like it that AM seems reliable on my tablet and prefer it to stay that way.
Wider palette than on earlier versions. Plus point.
Creeping features - where is this going - do we have coloured contacts too for business/pleasure? Minus point. A stable product that does first what is 'needed' gets my vote.
I don't have colours on my Thunderbird installation on PC, and don't miss them there.