Why won't my message send as an iMessage?

I'm trying to send my friend a message over iMessage from my iPhone to their iPhone. It keeps going green when I type in his name which means its a normal text and I don't know how to make it an iMessage

It either means your iOS software is out of days, or it's his. Just simply go to settings and check for an update and it will fix it.

It could also be he's not connected to Wi-Fi, or has no 3G or 4G. Same for you.

You boht have to have imessage on.