Blue means iMessage, and supports iMessage features.
Green means not-iMessage and supports SMS / MMS / RCS features.
Presumably when Apple rolls this out, both blue and green messages could be end-to-end encrypted. It remains to be seen whether Apple will add an indicator for RCS (and potentially iMessage) encrypted messages. I could imagine them being jerks about it and not indicating when green messages are just as secure as iMessage, or they adjust their messages that iMessage is more secure/private because Apple trusts their own key exchange more than RCS.
Green means not-iMessage and supports SMS / MMS / RCS features.
Presumably when Apple rolls this out, both blue and green messages could be end-to-end encrypted. It remains to be seen whether Apple will add an indicator for RCS (and potentially iMessage) encrypted messages. I could imagine them being jerks about it and not indicating when green messages are just as secure as iMessage, or they adjust their messages that iMessage is more secure/private because Apple trusts their own key exchange more than RCS.