Text Message bubble colors?

When I first got my iPhone last year, I was told that when I am texting someone else with an iPhone my chat conversation would have BLUE text bubbles. For those people I text that did not have an iPhone it would be a GREEN text bubble. If for some reason the text message won't go through while texting another iPhone user I will get a message that will say Send as Text Message and then that bubble would be green.

Here lately my text bubbles have remained green everytime I text my GF who has an iPhone and several other people I have texted that also have iPhones.

Anybody else notice this?