Apple's text message suddenly turned green.

What you want to ask is that Apple's short message suddenly turned green. What does this mean? It shows that the sending mode of iMessage has been switched to ordinary SMS.

Apple's green background when sending text messages, it will always be a green background, because this background is through the text messages sent by operators to let people know that ordinary text messages will display green. The color of the message received by the other party is gray background.