Folder color labels disappear after restart

Another magic spell to try: First, restart your computer. If the problem persists, try resetting LR's preferences:
https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/

LR sometimes soils its preferences file, and resetting it can fix all sorts of wonky behavior. That article explains how to restore the old preferences if resetting doesn't help.

Thank you @johnrellis -- I did as you suggested: deleted the helper.lrdata file, opened the catalog, added my color tag, restarted, and the color is gone again.

Didn't work. So I did it again... deleted the re-created helper.lrdata folder... still no luck.

Sadly, this did not work.

Try deleting the Helper.lrdata folder:

a) Do Catalog Settings > General > Show to open Finder / File Explorer on the current catalog folder.
b) Exit LR.
c) In that folder, delete the folder "<catalog> Helper.lrdata".
d) Restart LR, and it will rebuild the folder.

The Helper.lrdata folder caches information about metadata, keywords, folders, collections, and other things to speed the performance of LR and it can sometimes get corrupted. In recent versions, LR has used it more heavily to improve the speed of displaying metadata, and there have been a fair number of bugs with it.