u/Content_Owl1049, with both iphones on, signal, both on iMessage, if you are still getting a green bubble instead of the blue/imessage one, then you have 2 tricks go get the issue corrected:
1 st - Settings - Messages - Send as SMS - Off
- Send a message to that same person (works only if the other device is an iMessage capable one and that device does have data connection)
If that works, then you can turn SMS back on and you should be ok going forward:
- Settings - Messages - Send as SMS - On
If the above doesn't work, then
2nd - New Message Thread
- delete the message thread, then
- create a new thread and send a message to that same person
Usually if you delete the thread on the iphone that is affected, then the problem will go away as your device will keep sending iMessages ie. via your Data, instead of sending them as straight SMS messages.
If the above doesn't work, then
- do all the steps as on 2nd Option (see immediately above) on both devices
Make sure you update your post AFTER you've completed the above options with your results and let's go from there.