If the high contrast setting is turned on you won't see the fill colour.
There's information in the following MSKB article:

OFF: Changes to Fill Colour and Fill Pattern Are Not Displayed
http://support.microsoft.com/?id=320531


GBRAAT wrote:
When I fill a cell with color, it no longer appears as "colored" on the
screen, however if I print the document or go to print preview, the cell is
actually filled in with the right color. What have I done? The font colour
function works fine; just a problem with cell shade viewing.


--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html