Help! No longer able to change colored tags on files created by others

Highlight one of the files with the problem such as one from Person C that you cannot turn off and right-click and select Get Info ( CMD + i ). In the window that pops up you should see a "Green" tag in the text box at the top of the Get Info box. Click in the box and delete the "Green" tag with your delete key.. Close the Get Info window and the Green dot in Finder should disappear. You can then right-click and tag with a different color or if your Apps support it tag it when you save. In Finder there should be a tag icon on the toolbar at the top of Finder that looks like a sale tag. When you hover over the icon it will say Edit Tags. You can click that button and backspace over the "Green" tag as well. Note that the name of the tags might vary. Tags can be named and a color assigned. You can have more than one type of green tag.

Some of the confusion may stem from older macOS versions that used Labels instead of Tags. Tags can be named and you can apply more than one Tag, etc. The user interface changed quite a bit along the way. Tags are most useful for searching and categorizing and grouping files.