Event callback for a changed label/rating?

I'm working on a label bar that goes in the bottom navbar. It would be similar to the stars under each thumbnail and would indicate current label colors for selected files, and let you click to change label.

I have it working so far. It will indicate what labels are assigned to selected thumbs, and clicking a label assigns it to those files. However, I don't have a way to see when a label is changed manually.

Reading the documentation, there is apparently an event callback for certain kinds of infosets. Labels are in the core infoset so registering a callback for the current selection seems like the way to go?

I just need help in how to implement that callback.

My script is still in development and it uses a folder of PNG files for my iconbuttons but below are a couple of screenshots. Clicking a label changes the selection; selecting new files changes the indicators. If I can't easily capture a changed label event, I could always just skip the indicator images.

Ratings to be added later.