Having colored backgrounds might look ugly and thus immediately make that a non-option, and, even if viable, are still problematic if new messages are added while reading, as you'd need multiple colors for the reading pass the user's on.
Having to click a button repeatedly is also likely to be entirely unacceptable UX for most users, never mind that you're essentially recreating the receive-order message list, just without displaying it in a sanely traversable format.
This problem is hard enough with linearly added messages to the end, making it 10x worse is awful.