When you send someone with an iPhone a message and it goes as SMS and then the arrow turns green. It continues to stay green until that person replies back always by iMessage. They never reply by text. It is always iMessage.

In order for a message to be sent as an iMessage, both sender and recipient must be signed into iMessage and have an active data connection. If one person isn't connected to data for whatever reason, the message will get sent as an SMS (assuming you have that setting enabled).

That's exactly what happens and what it looks like when you have Send as SMS enabled in Messages settings on the iPhone and you go to send what would be an iMessage if not for iMessage service being unavailable for sending. The would-be iMessage falls back to SMS.

The other cat's reply will be iMessage if such service is available for him to send. Furthermore, if the other cat doesn't have Send as SMS enabled and iMessage is still unavailable on your end to relay his reply to you, it'll get backlogged with some kind of retry schedule and keep retrying variously periodically until it does come through at which point he'll get the Delivered acknowledgement. That's how you could have some would-be iMessages to certain cats fall back to SMS yet their replies are always iMessage. But that's a long shot as few cats wouldn't have that enabled.