Google Home and Google Home Mini's use LED lights at the top of the device to indicate what the device is doing. Most of the time it is used by the system to indicate state of the device or actions.
https://support.google.com/googlenest/answer/7073219?hl=en
Is it possible to use the LED lights to interact with the user or change the standard behavior to improve the experience for users? For instance, changing the LED's to a full red circle when the action hits a fallback intent.
No, 3P developers do not have any control over the LEDs that may appear on a smart speaker.