Meaning of red color in Android's Nine-patch drawables
It's too support layout bounds.
See https://groups.google.com/d/msg/adt-dev/z_iLJWyDQdo/lf4sR6CRl2cJ for more info.
I believe this thread has some discussion on the same topic.
This has got something to do with setting layout bounds and I believe its not documented yet.
The red lines are how you define optical bounds on a nine-patch. This feature was officially added in Android 4.3 but to date, its still not available in the support library. Read more about them in the 4.3 release notes:
http://developer.android.com/about/versions/android-4.3.html#UI