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

Before doing anything:


Quit the Messages app (Double press your home button and swipe up on the Messages app).
Go to Settings > Messages and Turn OFF and then back ON your iMessage option.
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:


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.
Try to send a message to your friend




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!