Yeah, I had to google it too:

"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."

So apparently iPhone users see text messages from us Android users in a green bubble instead of a blue one. And apparently they've been conditioned to think this is icky?