Why is the message sent between two Apple phones sometimes blue and sometimes green?

Blue SMS means that both parties are connected to the Internet and both use Apple phones, and the way to send SMS is "imessage".

The green message means that someone on both sides is not using an Apple phone, or someone is not connected to the Internet. In this case, an ordinary short message is sent. And "imessage" can be set in "SMS".

Extended data:

With iMessage, you can send and receive messages with any Mac user who uses iPad, iPhone, iPod touch or Mountain Lion, as well as send photos, videos, location information and contact information. If you have more than one Apple device, iMessage can keep an uninterrupted conversation on all devices.

In addition, you can send text messages, photos and videos to other mobile phones through the cellular network. You can even ask Siri to text you. Just say "Tell Xiaoli I'm on my way" and Siri will help you write and send messages.

It is reassuring to know that the information sent has been read and received. IMessage can tell you whether the message has been delivered, and if the receiver has enabled the read receipt, you can also see the read receipt message of the message. When someone is replying, iMessage can also display status information so that you can continue the conversation naturally and smoothly.