This is the original App from xda-developers. Don't accept fakes! You can get support here: http://forum.xda-developers.com/showthread.php?t=2516913
Just install this App and click on "Enable Battery Percent". Then reboot. You can uninstall the App afterwards. The setting will remain.
No root needed!
You can only see the battery percent text, if the battery is empty enough. (White text on white battery!)
FAQ: Do I need a rooted device? No, should work on any 4.4+ device :)
Can you change the text-color? Sorry, this is impossible with a simple app like this one, because the color is hard-coded in SystemUI.apk. You need to decompile that apk, edit one line of source code and then recompile it. Actually, I don't plan to do this, because I don't like to change system-apps. Also, the OTA-Updates from google would fail after that and it would require root.
After removing the app the percentage is still there. That's because the app changes a system setting. The app does NOT display the battery itself!
I uninstalled the App. Now I want to disable the percent display. OK, just install the App again, uncheck the setting and uninstall it.