Why is text green on the iPhone?

My boyfriend and i have the iPhone 4.sometimes our texts comes out blue which is totally fine BUT sometimes for a couple hours when i text him "the texts" comes out green. So i was wondering if they come out green cause he doesn't have signal or cause he turn off the phone.

Well sometimes this happens with me. When someone with an iphone texts me usually its green meaning your using regular text messages then i text it goes to blue. I guess its the cell phone carrier problem.usually tho if an iphone is off it will send a blue message then if then later it will say "sent as text message" thats when i kno their phone is off. So i'm guessin for you its jus small issue.

Why are my text background blue and sometimes green?

How do you know if your blocked? And what's the difference between being blocked and being on donnot disturb