why when i text my friend who had an iphone 7 plus is the message green and not blue

The text is green and not blue because at least one of you was not in an area that supports iMessage, and/or has Settings > Message > iMessage = "Off".

The message was sent as an SMS text message instead.

The message is green because of one of the feature of iphone that is iMessages. We can send meaage to other iphone user using this and its free of cost it requires mobile data or wifi. We can turn it off from the setting and there is other option too 'send as text message' that can be used in case either of the person's mobile data or wifi is not on.

Briansyddall wrote:

Messages from iPhone to iPhone are green message to android phone will be blue.

This is incorrect. iMessages are blue. They are sent between Apple devices (not just iPhones) if both users have iMessage enabled and have an active data connection. SMS messages, such as would be sent to an non-Apple device, are green.

Hi

Messages from iPhone to iPhone are green message to android phone will be blue.

Cheers

Brian