It is easy to switch. The default icon theme is called "hicolor" and that is where applications install their included icons.
So you can override it if you put a file here in your home directory:
But after seeing the Faenza theme, I think the brown Kupfer icon looks great. The Faenza theme can also install a monochrome kupfer icon for the panel for you.
So I'm no longer that positive to switching Kupfer's icon to monochrome everywhere.