sheet background color suddenly changed to light green from normal
 

I have an EXCEL file with over 17,000 records that I update every month.
Today, while adding additional data (copy/paste) from another file, my
background color suddenly began to change from white to a very light green.
The added copy/paste data was white, as usual. The Print Preview shows normal
white and printouts of other sheets with pivot charts, tables, etc. print
normally.
All sheets in this file are now this very light green. I changed the
background color to No Fill but that doesn't work.
Any ideas?
Thanks,
Dan