Here's the link to download the app from Google Play - Grayscale+
In order to be able to use this app, you should grant secure settings permissions through adb
adb -d shell pm grant com.berenluth.grayscale android.permission.WRITE_SECURE_SETTINGS
If you don't have adb yet, here's a guide to install it https://www.xda-developers.com/install-adb-windows-macos-linux/