How to get an iMessage conversation to go back from green to blue?

Settings

Before doing anything:

1. Quit the Messages app (Double press your home button and swipe up on the Messages app).
2. Go to Settings > Messages and Turn OFF and then back ON your iMessage option.
3. Now open back Message and try to send a message to your friend 's iPhone, but make sure to use the button at the Top-Left corner to find your friend and start the conversation, don't just open your recent chat with him/her.

This should have reset your iMessage settings, so you should be able to send iMessages to your friend now.

Restart

If the process above didn't solve the issue, you may want to:

1. Restart your iPhone, long press the Sleep/Wake button until you see the Apple logo again, then release, OR you can long press until you see a slider which can be used to turn OFF the iPhone, once done that, press again the Sleep/Wake button to turn the iPhone back ON.
2. Try to send a message to your friend

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I'm sure you already know the difference between iMessage and normal SMS/MMS, although that, I'll leave you a link to the Apple Support page about the difference, have a great day!

• Close your Messages app (double click the home button then swipe from bottom to top on the app)
• Restart your phone.
• After the phone boots back up, open Messages.
• Initiate a message to the other person via the Compose button.

Restart your iPhone by holding the Sleep/Wake button until you see a slider. Slide to shutdown your phone and wait until it is fully powered off before powering it back up.

I read many comments to try and solve my inability to use iMessages with my friend, They all overlooked what my problem was. I turned iMessages on and off in Settings, restarted my iPhone 7 and deleted the entire past message thread between us. The problem was, there are two ends to a message. My friend simply didn't have iMessage turned on on his end.

Send a photo or attach a screenshot to the message. That pushes data through the pipe and will show you that the iMessage network refreshes. This usually fixes the issue.