You can also use Shizuku to grant the permission. Just install Shizuku and aShell from F-Droid. Set up Shizuku and if you don’t know how to do it just watch a video. Then grant Shizuku permission to aShell and type this command [pm grant ly.com.tahaben.farhan android.permission.WRITE_SECURE_SETTINGS] after that open Farhan and you should see the gray-scale working. But unfortunately this only works in android 10 and above.
You can also use Shizuku to grant the permission. Just install Shizuku and aShell from F-Droid. Set up Shizuku and if you don’t know how to do it just watch a video. Then grant Shizuku permission to aShell and type this command [pm grant ly.com.tahaben.farhan android.permission.WRITE_SECURE_SETTINGS] after that open Farhan and you should see the gray-scale working. But unfortunately this only works in android 10 and above.