Why is the message sometimes blue and sometimes green when sending short messages?

The short messages displayed in green belong to the short messages of traditional operators.

Blue SMS is a service that Apple provides for iOS devices to send information through the network for free.

There is no limit to the sending of green SMS, and it can be used regardless of the model of the mobile phone. This short message is from traditional operators, namely China Mobile, China Telecom and China Unicom. In addition, you can also see a prompt with the word MMS in the SMS text input box.

Blue SMS can only be used when both parties use iPhone, and both parties have opened iMessage service and have network connection.