I wonder what the use case was for fading the chat bubble color as you go later in time? I haven't seen iOS 7 in action yet but I wonder what happens when you scroll up: how do the colors change?

At a glance the earlier messages I would have sent look harder to read.

As the messages move closer to the bottom of the screen they become more opaque. So as you scroll, the things that preside close to the bottom of the actual device, that are on screen at that moment, will be totally opaque.