Why is my Iphone 3G texting bubble blue?

I have a jailbroken iphone running on ios 4.2.1 and somehow my texting bubble is blue? Does that mean i'm using imessage or is that the old color for the iphone 3G? Imreally not sure and i would like to know because using imessage would be terrific!

It's probably imessage. If when you send a message underneath it says 'delivered' it means you have imessage. If not you just have a cool phone