A support page on Apple's website says the green text bubbles come from users who send Short Message Service (SMS) messages. Apple's iMessage is only available for its products including iPhone ...

Blue bubbles versus green bubbles - in the world of texting, it's the difference between iPhone users and people with Androids. It's like a bubble culture war. People have been called out.

Furthermore, the article quotes a 2013 email from Apple’s Craig Federighi, in which he presents his opposition to an internal suggestion to bring iMessage to Android.

Apple confirms the blue versus green bubble battle won't be over just because of RCS. The blue iMessage text bubble is meant to tell the user when they are talking to another iPhone user.

Apple has blocked Beeper Mini, which lets Android users send blue-bubble texts to iPhones. The tech giant said the app "exploited fake credentials" and posed a security and privacy risk.