Alternative Notification System for Macos || Program or CLI to Display Image/Emblem/Icon at corner of screen (no text or box)
The premiere notification third party notification system for macOS is Growl:
https://github.com/growl/growl
The project was just recently marked as retired (less than a week ago), but you can still download the code and use it. It is very easy to use Growl to trigger a notification from a shell script.
If you can live with the built-in macOS notifications, you can use a command like this from your shell script:
osascript -e 'display notification &quot;Mode change&quot; with title &quot;Mode change&quot;'

This uses the built-in AppleScript function for displaying a local notification. You can ofcourse change the text to be whatever you want, like &quot;Bindings turned on&quot;, and so on.
If you want to write your own application to display the notification, you could start looking at this sample application code from Apple:
https://developer.apple.com/library/archive/samplecode/FunkyOverlayWindow/Introduction/Intro.html
It is basically an overlay window that appears on top of every other window at all times. If you make it small, you could use it to display a color when you want, and let it be fully transparent at other times.