kbf parent
Green means cellular, blue means internet. This same color distinction is used in other places in the UI, such as control center and settings.
Nope, it means SMS vs iMessage specifically.
> If you see a green message bubble instead of a blue one, then that message was sent using MMS/SMS instead of iMessage.
https://support.apple.com/en-us/105087
> iMessages are texts, photos, or videos that you send to another iPhone, iPad, iPod touch, or Mac over Wi-Fi or cellular-data networks. These messages are always encrypted and appear in blue text bubbles.
MMS is email-style MIME messages retrieved over HTTP on a data connection. Push notifications for MMS are delivered as a special SMS, but otherwise it's far far closer to internet than cellular, and it's green.