This case occur in NVDA2015.2 and later.
STR:

1. Open the attachment.
2. Move to cell I6.
   Press NVDA+f
   NVDA 2015.2 report: Black on Coral
   NVDA2015.1 report: black on orange
   sighted person say: black on orange
   note: Here NVDA detect-background matches with Excel Fill Color. I just curious why sighted person say "this is black on orange". Also, it seems difficult for a blind user to know that color coral is close to orange.
3. Move to cell F13.
   Press NVDA+f
   NVDA 2015.2 report: Black on Gold
   sighted person say: black on yellow
   note: In this case, Fill Color "orange" is selected.