Different-colored bubbles on iPhone text messaging?

I'm a new iPhone owner and when sending text messages, some bubbles with my sent texts are in blue, others are in green. Anyone know what this means?

Blue=iMessage, between you and another Apple user
Green=Text message, between you and someone without an Apple product

Blue - iMessage, internet based, between Apple devices
Green - SMS message, carrier based, any phone

Green=non-iOS user
Blue=iOS user
Blue texts are free over Wi-Fi, by the way.

  • Why do my text bubbles sometimes change color on the same number? I understand that it's blue when using iMessaging and texting between iproducts, and green for standard SMS messaging. However, when I text a friend of mine with an iPhone, while most of the time the text shows up as blue, occasionally it shows up as green. Is there a reason for this?
  • Why can't my iPhone send or receive pictures in text messaging? My dad bought me the iPhone 4S unlocked. I got the prepaid service at tmobile which was 50$ unlimited talk text and web. I can't send pictures during text messages and I can't receive them. If someone sends me a picture I don't get it, it's just blank. The little camera icon next to the text area to type is light grey indicating I can't press on it. I can only send or receive during iMessage. Please tell me why this is happening.
  • Why are some of my text bubbles dark bule? I have just realized that some of my text to and from my husband are dark blue. He doesn't have a iPhone and he has a different phone carrier as well. Can anyone tell me why it's doing that?
  • What does the green text bubble mean while text messaging with Iphone? I have an Iphone5c and my friend has an iphone 4. Whenever we talk all my messages are in blue but I sent a text today and it was in green? Should my Wi-Fi be on or no? P.S. I have ios 7