Someone has probably applied a conditional format to the workbook. Try going to format, conditional formatting, delete, and then check all three boxes, click ok, click ok again. If the problem still persists, please advise if it is across all workbooks or just this one.
Thanks ever so much! I'd read about this "high contrast" option, but I coudn't find it anywhere in the "appearance" settings, where I would have expected it to be. Anyway, problem solved. I'll now be able to sleep. Merci beaucoup!
Funny thing is that I knew I'd seen this problem before, but I had thought the cause was setting the Print in Black & White option in PageSetup Dlg box. When I tested that theory and it didn't pan out, I did a search here and found that thread. So I learned something too.