Greyscale

Before using this Application, you need to use "adb tools" to grant permission, copy below command and exec it on a PC.

adb shell pm grant com.fei_ke.greyscale android.permission.WRITE_SECURE_SETTINGS

About "adb tools": https://www.xda-developers.com/install-adb-windows-macos-linux/

Source code: https://github.com/fei-ke/Greyscale