Apple’s iMessage service typically works very well. However, on rare occasion you may find messages to one contact may have changed from iMessages (blue background) to SMS text messages (green background).
If you are unsure about the difference between an iMessage and an SMS text message, this article provides additional details. https://www.ianswerguy.com/green-blue-iphone-messages
Messages Changed From Blue to Green
To send an iMessage, your iPhone must be connected to the Internet, either by Wi-Fi or a cellular data network. Without an Internet connection, your iPhone will not send iMessages. If you happen to be in an area where there is no Wi-Fi or cellular data network, messages that would normally be sent as iMessages may be sent as SMS text messages.
If the recipient disabled iMessage on their iPhone, you will not be able to send an iMessage to them. Instead, the message will be sent as an SMS text message.
If you changed your Apple ID password, you will need to sign in with your new password to reauthorize your iPhone to use your iMessage account.
If your iPhone continues sending SMS text messages instead of iMessages to an iPhone user, you may need to reset iMessage on your iPhone. To reset iMessage, Go to Settings -> Messages -> Send & Receive and uncheck the email addresses in the “You Can be Reached by iMessage at” section.
Then, tap your Apple ID at the top of the screen and select Sign Out.
After signing out, make sure the slider for iMessage is set to the off position.
Next, reset the iPhone by pressing and holding both the Sleep/Wake button and the Home button for about 10 seconds. While you are holding the buttons, the screen will go blank. Continue holding the buttons until you see the Apple logo. You can release the buttons after you see the Apple logo appear on the screen.
Once the device restarts, wait 5 – 10 minutes, then turn the iMessage slider back on and sign in with your Apple ID. The iMessage activation should proceed normally. Once iMessage is activated, go back to Settings -> Messages -> Send & Receive and make sure any email addresses you want to use for iMessage are selected.
What If the Issue Persists After Resetting iMessage?
If you reset iMessage on your iPhone and messages are still being sent as SMS text messages instead of iMessages to another iPhone user, that person may have been the one that needed to reset iMessage. Have your contact reset iMessage on their iPhone.
If you have additional questions about sending and receiving messages, please see my other articles discussing text messages and iMessages.