The white on green is only used for the SMS messages you sent in the conversation. Incoming messages are always black on grey, for SMS and iMessage both. If t

The white on green is only used for the SMS messages you sent in the conversation. Incoming messages are always black on grey, for SMS and iMessage both. If the color scheme actually impacts legibility, it would only affect messages which you yourself wrote. It wouldn't have any effect on legibility of messages other people wrote, where that really matters a lot more.