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

The fill color, the fill pattern, or the line color of a WordArt or
AutoShape object in an Office document does not change
http://support.microsoft.com/default.aspx?kbid=833258


Remus Heber wrote:
I have set the fill and the font Icons in my Excel toolbar. However, when I
click to the fill to a cell or cells, and when I click to change the color of
the font, it remains white, the normal background.


--
Debra Dalgleish
Contextures
http://www.contextures.com/tiptech.html