How can I capture change events for the contents of the Swatches panel?

When a swatch is added to or removed from the Swatches panel in the document workspace, I need to capture this event.

Which protocol should I observe?

In my observer implementation, I attached like this:

InterfacePtr<ISubject> wsSubject(workspaceRef, IID_ISUBJECT);
wsSubject->AttachObserver(
    ISubject::kBothAttachments,
    this,
    IID_ISWATCHLIST,
    IID_SELECTION_CHANGE_IOBSERVER
);

 However, in my Update() function:

if (protocol == IID_ISWATCHLIST)
{
    // never reached
}

I cannot receive any notifications at all.

What is the correct way to observe swatch creation and deletion events in the workspace?