How to detect color panel's color change in Mac Catalyst app?

In Mac Catalyst, there is a default toolbar item that shows a color panel.

https://developer.apple.com/documentation/appkit/nstoolbaritem/identifier/1531463-showcolors

It shows up just fine, but I can't find a way to detect a color change from this color panel. Does anyone know how to do this?

When the toolbar item is clicked, an instance of NSColorPanel is shown. The selected color can be accessed from NSColorPanel.color, but we don't have a reference to the shown panel. Even the class NSColorPanel isn't visible to Mac Catalyst apps.

Fortunately, there is a notification that is posted when a color is selected in a color panel. So, all we need is to observe that notification to get a reference to the NSColorPanel instance, and then access its color property:

NotificationCenter.default.addObserver(forName: .init("NSColorPanelColorDidChangeNotification"), object: nil, queue: nil) { notification in
    let color = (notification.object as? NSObject)?.value(forKey: "color") as? UIColor
    print("Color changed", color)
}