No root required. This is the original App from XDA-Developers. You can get support here: XDA-Developers.
Just install this App and click on "Enable Battery Percent". Then don't forget to reboot. You can uninstall the App afterwards. The setting will remain.
Attention: On Android 4.4, you can only see the battery percent text, if the battery is empty enough. (White text on white battery!)
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.