Shared spreadsheet changes color when opened ny another user. Why
 

One user adds information to a shared spreadsheet and then closes it. Another
user opens it to add information but now the spreadsheet is colored red. It
start to happen this week.
I asked both user to close the spreadsheet.
I had first user re-save the spreadsheet and then had the second user open
it. Lo and behold, the color was gone but today it is happening again.
Any thoughts.