There is a way to do this. It's not simple, and can't be done via the Earth Pro interface, but can be done by editing the KML (manually or programatically) to add a "List Style", and creating an "icon" that looks like your polygon list symbol.
First, you have to go in and add a
<Style><ListStyle>... section to each Folder element, or as a shared style referenced by the appropriate folders. List Styles let you customize how each folder is shown in the Places list, including what type of folder it is (checkbox, radio button, hide contents, etc.) as well as the background color (not recommended to change this), and the icon shown. For folders, you can specify two different icons, for the open state and the closed state (for NetworkLinks there are many additional states). If you add a section similar to what's below, you can customize your folder icon:
If your folder is not locked closed with "checkHideChildren", then you'll probably want to add a second
<ItemIcon> section for the open state. And of course, if you have other style types applied to the folder, or in the shared style, they can be included in the same
<Style> section, before the
<ListStyle> (per the spec ListStyle goes last). More information on ListStyle is available in the documentation: https://developers.google.com/kml/documentation/kmlreference#liststyle
Unfortunately the polygon symbol shown in the list view by Earth Pro is not available as an icon file, as far as I know, so you'll have to make your own. Recommended icon sizes are 32x32 or 64x64 pixels. If you want one that looks just like the polygon list symbol, then what I would do is take a screenshot of Earth, crop out just the polygon icon that you want, adjust size if needed, save it as a PNG file, put it on your server for hosting, and insert the URL into the sample style above. You'll need to make one for each color.