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
Fixed FC on some device.
9Apps 4.9
