There is an interesting application called Twilight which appears to simply draw over all applications with a reddish filter to remove blue light during nighttime hours (presumably to help sleep).
I’m interested in doing something similar, but not necessarily for sleep purposes. I’d like to draw text or a watermark over everything on the phone. How can I do this (draw an Activity or something like it over everything) in Android?
You should look at Android System Overlay which pretty much draws a view on the root on Android Window Manager.
You need “android.permission.SYSTEM_ALERT_WINDOW” permission in your manifest.
You might also like to look at Window manger