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

When you both got 3G or Wi-Fi enabled, the text bubbles will turn blue and grey. But when one of you got 3G or Wi-Fi disabled, the text bubbles will turn green and grey.