Meaning of red color in Android's Nine-patch drawables

According to the Android documentation, we know that the left and top black line define the stretchable area, and the bottom and right line define the content area.

It's easy to understand, but I found the below image named textfield_activated_holo_dark.9.png from android-17 framework. What is the purpose of the red colors in the lower-left, lower-right and top-right corners?

enter image description here

Solution

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.