A free app for Android, by Vegard IT GmbH.
Gray-Switch is a handy tool for converting your Android device's display mode from its original bright color to a more comfortable and less distracting grayscale. The app can be easily configured to automatically change the display mode when the battery charge reaches a certain level, when the screen is idle for a certain period of time, and/or when the device is connected to an active charger.
To use Gray-Switch, you first need to install it, then launch it from your device's main menu. After that, configure it to match the behavior you want. You can use the built-in settings to set the threshold level for the battery charge, the period of time when the screen should be idle, and the time when the device should be connected to an active charger. You can also exclude specific apps from the display mode change.