Why are my iMessages sending green to another iPhone

In order for a message to be sent as an iMessage, both parties must be logged into iMessage and both parties must have an active data connection. So, if your friend doesn't have a data connection (happens to me when I'm underground on public transit), the message might still go through but as an SMS, which doesn't require a data connection.

If the person has an intermittent data connection, iMessage isn't always very graceful about switching back and forth so, sometimes, even if the person is once again connected to data, the message may still go as SMS for a while. People who have extremely intermittent data access are often better off staying with SMS

Are you certain that the iMessage button is turned on under Settings>Messages on your device? Here are some other suggestions:

If you see a green message bubble

If you see a green message bubble instead of a blue one, then that message was sent using MMS/SMS instead of iMessage. There are several 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. To check if iMessage is turned on for your device, go to Settings > Messages > iMessage.
• iMessage is temporarily unavailable on your device or on your recipient’s device.

Learn the difference between iMessage and MMS/SMS messages.

This is the support article that the info above is from: If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

Cheers,

GB

Does the other phone have iMessages enabled?