It is not possible to clear or set the default fill colour of an icon when it has been linked to a property in a component and the default icon file is not set.
I had a coloured fill applied to an icon which was linked to a property and I could not find where it was set. There was no fill set on the component property, no custom CSS. After much head scratching I found it when I unlinked the property, then added a default icon. I then saw the default fill colour set, removed it and then relinked the property.