[MINOR] Color nodes by directory

[shunlog]

Have you checked whether this feature is not already on the project board?

Yes

Is your feature request related to a problem? Please describe.

I have a directory for bibliographic notes and another for "permanent" notes about ideas. I would like to be able to color nodes based on directories so that it would be easy to tell which nodes are about ideas and which represent sources (books, articles, etc.).

Describe the solution you'd like
I imagine there could be a setting called "Directory colors", very similar to the existing "Tag colors" but instead of assigning colors to tags, you would assign them to directories.

A color assigned to a sub-directory should probably take priority over a color assigned to its parent directory.

Describe alternatives you've considered
I could simply use tags instead of directories (e.g. "bibliography" and "permanent" tags), but I would really prefer not to do that. I think tags should only be used when a note could be used to multiple of them. In this case, a note be either a bibliographic note or a permanent note.

Additional context
Take a look at these nodes here:

Can you immediately tell which are bibliographic notes and which are simple notes?
That's why I want to assign them colors.

I have also thought about emphasizing them by size or opacity, but that wouldn't be that straightforward.