Font Color turned Red on all saved Excel files

Without seeing your setup, it sounds as if you have some code running ON SAVE or ON OPEN.

If it's happening just for you, I suggest finding the PERSONAL.XLSB in the XLSTART folder, and removing it by copying it out of there, perhaps to your documents folder for future inspection. For more info, check here: https://support.microsoft.com/en-us/office/copy-your-macros-to-a-personal-macro-workbook-aa439b90-f836-4381-97f0-6e4c3f5ee566

If it's happening across an organization, well...there's a gremlin running around somewhere.