If it were up to me, encrypted RCS would be blue and not-encrypted RCS would be green.
Grey and Green means not encrypted. Simple.
> 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.
You could say "blue means it supports link embedding; green means it doesn't" and it would be just as true.
Personally, I strongly suspect that Apple views it as "blue means Apple; green means NIH."
I can’t imagine all of the carriers cooperating on key management. That’s probably why some carriers have opted to use Google’s solution instead of their own. I would guess encryption isn’t an option if your carrier isn’t in this group.
FWIW, the 350+ page standard is here: https://www.gsma.com/futurenetworks/wp-content/uploads/2019/...
I find this absolutely hilarious and almost beautiful: Google has been harassing Apple to implement RCS because it's an open standard and because its users feel green bubbles on iMessage are exclusionary. Now Google has implemented a proprietary protocol on top of RCS that only works with its messaging app, and only messages sent between users of that app appear in a darker blue color with a special lock icon.
So Apple will ship the RCS standard in iMessage, and communication between Android users and iMessage will be sent using RCS, but iMessage users will appear to Android users in the lesser light blue bubbles alongside the dark blue bubbles only given to Android users with Google's proprietary app! Huzzah!