I'm skeptical of the implementation, given iMessage. I'm sure Android users RSVPs will show up in a green toxic fume cloud or some such.
As the iOS user, it is your own messages that are green or blue depending on whether it was sent using iMessage or SMS. It's useful feedback about whether your message was sent on a reliable channel.
I know it became a whole thing and that Apple has allowed it to remain as such. But it's not really an apt analogy.
I don't know how long RCS has been around, but my impression is most or all of my messages until recently were SMS.
iMessage now says "Text Message - RCS" or "Text Message - SMS" in the text entry box which is better than the green/blue bubble thing (though it does still have that).