Individual apps can be set to display in grayscale in Android Q, according to the Android Developers website. It's hard to tell how the new functionality will eventually be implemented, though, as it doesn't seem to be present in the newly-available beta release.

You can already set your screen to grayscale using the Wind Down settings in the Digital Wellbeing app; removing color from your screen should, in theory, discourage use of your phone. But it's currently an all-or-nothing setting, affecting all apps as well as your home screen. It's possible Digital Wellbeing will receive an update to enable per-app control on Android Q in the future.

The current grayscale settings in Digital Wellbeing.

The option might also be activated from within individual apps, requiring developers to make use of a new API to enable the setting. Whatever the case may be, per-app grayscale settings don't seem to be available just yet.