Currently, Read and Selected messages are almost exactly the same shade of gray. If both types are onscreen at the same time, you can (just barely) tell them apart. But if only one or the other is onscreen, there's really no way to tell.
See the screenshot attached. One has a Read and Unread message, one has a Selected and Unread message. Can you tell which is which?
Maybe use a checkmark for Selected messages (like every other program everywhere) or failing that, at least colour one of them differently (as K9 has done on some releases).