In addition to folder colors, which we could have more of, would it be possible to add simple shapes to folders for more customization? For example, overlay a small plus sign, cross, circle, square and more on top of the folder icon.

With this we could label folders not only as green or blue, but green, blue, green with triangle, blue with plus sign, etc.

Since folders and backgrounds can be various colors, the icon might need color options too. Settings such as transparent (ie the shape will be cut out of the folder and take the color of the background) or taking the inverse of the folder color would be useful along with manual selection.