Wait, what? Android user here so I might just be confused, but my impression always was that it's the received messages that are different, hence the whole blue/green bubble debacle, and Android users being bullied for having green bubble texts.
Received messages are always grey. It's sent messgaes that change colors. So a message from an Android, iPhone, windows phone, feature phone, etc. will always appear with a grey bubble. A sent imessage message to an iPhone user will be blue. A sent SMS text message to a iphone, android phone, feature phone, etc. will be in a green bubble.