You could use a directory-local variable. Relevant documentation node.
To use this, you create a file
.dir-locals.el in the root of each project dir with contents like:
((nil . ((my/is-local . t))))
Then in your config you need to do two things, you need to set the variable as safe so Emacs doesn't prompt you about it whenever it loads a file in that directory, and you need to modify your display however you want based on it. Below is an example:
;; Mark any boolean value as safe.
;; See https://stackoverflow.com/q/19806176/4851078
(put 'my/is-local 'safe-local-variable #'booleanp)
;; Set unfontified text in buffers from the local repo to dark blue
;; See https://www.emacswiki.org/emacs/FacesPerBuffer
(when (bound-and-true-p my/is-local)
(buffer-face-set '(:foreground "#003")))))