What's the difference between the green and blue dialogs of Apple SMS?

What's the difference between the green and blue dialogs of Apple SMS?

What do I think is the difference between the green and blue dialogs of Apple SMS?

The green dialog box indicates that the short message is sent through the operator's network, which is the traditional short message service.

The blue dialog box indicates that both the sender and the receiver use Apple devices that support iMessage and are connected to the same Wi-Fi network.

The difference between the green and blue dialog boxes lies in the network connection between the sender and the receiver.