How to change the text bubble on Iphone 4s?

I have an IPhone 4s and I text all the time.sometimes Imessages sometimes just Text Messages but I'm getting a little tired of just the plain green and white for texts… Is there a way to change those colors? It's ok if not I'm just wondering…

My friend do this, I can't remember the name of the app, but it's free. Go into your App Store and type in "colour text" and see what comes up!