Solve! The messages between apples are blue at first, and then the blue messages turn green again?

Display green indicates that this message is sent and received by normal operators, and display blue indicates that this message is sent and received by iMessage function.

The messages displayed in green belong to the messages of traditional operators, that is, they are generally charged according to the number of messages. There is no limit to the sending of this short message, and it can be used regardless of the mobile phone model. This kind of short message is used by traditional operators, namely mobile, telecom and Unicom. In addition, you can also see a prompt with the words "SMS/MMS" in the SMS text input box.

Blue SMS is a service provided by Apple for iOS devices to send information through the network for free, that is, iMessage information, whether using wireless network or mobile network. This kind of short message can only be used if both parties use iPhone, and both parties have opened iMessage service and have network connection.

Extended data

Similar to MMS, iMessage not only allows users to send text to other iOS and OS X users via Wi-Fi, 2G, 3G or 4G LTE, but also can transmit documents, images, videos, locations, contact information and group information, thus providing standard SMS and multimedia SMS services for most devices with iOS 5 and above.

IMessage is implemented on iPhone, iPad, iPod touch running iOS 5 and Mac running OS X Mountain Lion through the "Messages" program. Owners of these devices can register multiple email addresses on Apple's network. If the communication operator supports it, the iPhone owner can register an Apple account with this phone number.

Baidu encyclopedia -iMessage