Sending text messages is blue and green for a while.

Conversion from message to SMS: The blue SMS bubble indicates that the message was sent through Apple's iMessage service, and both parties need to use Apple devices and log in to iMessage account. If the other party uses a non-Apple device, such as an Android phone, the message will be converted into a green short message and sent through the operator's network. Unable to connect to iMessage service: If you encounter connection problems when sending iMessage, your message may be converted into SMS and sent through the operator's network. This may be due to network problems, setup problems, or the device of cannot access the iMessage service. Different contact settings: If a contact's iPhone can't connect to the iMessage service, or their iMessage service is turned off, the message sent to them may be sent directly by SMS. Poor network condition: a green dialog box may indicate poor network condition, and the system will automatically convert the information into short messages to send.