How to dynamically change color of SubGraphExpanderButton

I am trying to changge the color of the SubGraphExpanderButton on certain events. I can do this with other parts of my node with code like this:

    shp = node.findObject(NODE_SHAPE);
    if (shp !== null) {
          shp.fill = fillColor;

However, I haven’t had success with this code for SubGraphExpanderButton:

   var shp = group.findObject("SubGraphExpanderButton");
    if (shp !== null) {
        shp.fill = "red";

Thanks in advance!

First, to understand how the “SubGraphExpanderButton” is defined, look at its definition at
Note that “SubGraphExpanderButton” is a “Button”, so you should look at that definition too, earlier in that same file.

I’m guessing that you want to look for the Shape named “ButtonBorder” so that you can set its Shape.fill property. But you might also want to find the “ButtonIcon” Shape so that you can change its fill, stroke, or background.