Let's suppose that I created a new tag "custom tag". Is there a way to not only delete said tag, but also relabel all files which were previously labeled with that tag with a default one, like blue, red, green... automatically?
Delete the custom tag last…
First, open a new finder window & from the sidebar set it to show the files with the tag you wish to replace.
Select all files in there - Cmd ⌘ A - then right click & add your desired default tag.
With them all still selected, right click & remove the custom tag. They will vanish from the window at that point, but they will have moved to the new tag you assigned.
Then, in Finder prefs > tags, click the tag you want to delete & hit the - button underneath.
Tried to squeeze all that into one explanatory picture…