It’s commonly misremembered that green bubbles were invented for Android and other non-iMessage users, green being chosen explicitly to evoke the color of the Android logo. (It’s also misremembered that it’s the incoming message bubbles that are green, thus branding the sender as an Android user.) But SMS messaging on iPhone used green bubbles for outgoing messages from day one.
This brought up an interesting thought regarding Apple’s Messages app, why does it only support iMessage and SMS? iChat had support for AIM, Yahoo Messanger, ICQ, and XMPP. Why hasn’t Apple gone beyond the blue and green bubbles, introducing support for additional protocols with more message bubble colors?