But let's run with that for a moment, and assume many people do in fact find that more difficult to read. I still have trouble calling that particularly hostile given that it's sent messages, received ones are the same color no matter what.
I'm more open to the green vs blue argument than the old-green vs new-green one. Apple definitely wants you to know you're using iMessage. It just happens to be useful for me as a customer, too -- I'm glad it's prominent when I send a text message instead of an iMessage. It aligns my expectations for what features will work in the conversation.
For small developers there's checker tools and simulators, but Apple is huge and has a responsibility to get this right.
Most accessibility problems aren't things that those without some sort of sensory disability (beyond mild long-sightedness) can detect easily - at least, without using tools to do so.
Surely though there is some sort of "accessible" mode you can put it into that does improve the contrast?
In fact, it is much better than what iOS6 had.