Why does my iMessage turns green ?

Question

I have a dilema I will send someone I know we both have iPhones and when I sent her a message this time it would not say delivered them a few seconds to a minute it turns green and send as a text message and for a while the little bubble at the bottom would say text message and I checked back later on and it would now say IMessage at the bottom why is that 

Answer 1

Sometimes you dont have wifi for imessage, then it turns green.

Answer 2

Thats because one of you has sms messaging turned on and it needs to be turned off cause its for non iphone users only(example one of you send a message to someone with android)