If your iPhone messages are green
Why are your iPhone messages green? RCS and MMS/SMS messages appear in green text bubbles on your device. Find out how to see blue bubbles instead.
Why your messages appear in green bubbles instead of blue bubbles
If you see a green message bubble instead of a blue one, then that message was sent using RCS or MMS/SMS instead of iMessage. There are several possible reasons for this:
The person that you sent the message to doesn’t have an Apple device.
iMessage is turned off on your device or on your recipient’s device.
iMessage is temporarily unavailable on your device or on your recipient’s device.
You may need to update your settings for Messages after you set up a new device.
Find out the difference between iMessage and RCS or MMS/SMS messages
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Turn on iMessages so your messages will appear in blue bubbles when possible
In the Settings app, tap Apps.
Tap Messages.
Turn on iMessages.

3. If needed, tap Send & Receive to choose whether to start new conversations from your phone number or your email address.