Indeed, all you can do is learn and memorise what they represent. But a square filled with a solid color can do the same ("yellow is the hard disk thingy"), and that would actually be more glanceable and quicker to distinguish.

If your icon loses to a yellow cube, it is not a good icon.

Back in the day, when you could still have custom icons in the sidebar of the Finder and file dialogs (well, Snow Leopard…), I used colored spheres as identifiers for project folders, for minimal 1-bit attention. Which may be a proper use case for such a thing. (There's a clear relationship, but this is also subject to change.)