Why aren't my iMessages in a blue bubble?

Usually when iMessaging/texting somebody else with an iPhone, your messages are in a blue bubble. I have a friend who also has an iPhone, but when I text her my messages aren't in blue. Is there a reason for that?

Mines does that sometimes too. Its stupid

She needs to have her iMessage on in order for you to iMessage her.

I just switched iPhones and I noticed that the ones in a blue bubble are when I'm corresponding with someone else who also has an iPhone. When I talk to people that I talk to who don't have iPhones it's always in a lime-green bubble.

Both of you have to have imessage on