It's not the color of the message bubble, that's just a shorthand for the actual problem: it's that iMessage has more functionality than SMS, and inviting a SMS user to a iMessage chat works but degrades the experience for all participants.
It is both awkward (for instance, using a reaction generates a "so and so liked your thing" message) and insecure (now every word in a chat gets blasted out over the known-insecure medium).
Those reactions seem to work for me in Aus., on Android.
And as for group chats, you should understand that if you start one on iMessage, you shouldn't be surprised that it doesn't work cross-platform. It could, but Apple deliberately doesn't want to make an iMessage app for Android.
Understand the implications of using a proprietary system.
Its getting to the point now where I'm never going to buy an iPhone simply because getting excluded over a green bubble seems like a good way to filter stupid people out of your life.
