The background color of an SMS text message the user sent in an iPhone, starting with iOS 7. Green is for text messages, while blue is the background of iMessages. The colors differentiate the app being used, because although the words "Text Message" and "iMessage" are displayed in the text box, they are very faint gray and disappear as soon as something is typed.
A Green Bubble
The green color was added to differentiate SMS texts from iMessages, because the grayed "Text Message" does not stand out.