Why are some messages sent by Apple's mobile phone green and some blue?

The blue message on Apple's mobile phone indicates that both parties are connected to the Internet and are using Apple's mobile phone. 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".

Materials/tools: iPhone8.

The specific operation includes the following steps:

First, open "Settings" on the phone desktop.

2. After opening Settings, find the "Information" column and click Next.

3. Click the button on the right side of "imessage Information" on the "Information" page to open this function.

4. Turn on this function, and two Apple mobile phones can send messages through "imessage" when they are connected to the Internet, thus reducing the SMS cost of mobile phone operators.