Custom folder icons via Thunar

I am not sure sure about all GNU/Linux systems but on a system with an 
XFCE desktop environment, you can currently customize a folder icon on a
FAT-32 formatted partition by simply adding either a JPEG or PNG file to
the directory as "Folder.png" or "Folder.jpg" using the Thunar file-
manager. You can also use "Cover.jpg" if the folder is to represent 
something that would have a cover, such as disc-based media. Also, 
"Folder.png" can have an alpha channel, i.e. "transparency." These 
things are possible due to the "tumbler" thumbnailing service.

Beyond that, the Thunar file-manager has built-in features via right-
clicking a folder and going the "Properties --> Emblems". These emblems 
are installed along with certain icon theme sets and changes do not 
transfer and are only related to the current "owner" of that folder or 
file.