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

Changes to the fill color and the fill pattern are not displayed in
Office programs
http://support.microsoft.com/?id=320531


Nigel wrote:
Filled some cells with a colour, but am unable to see colour in worksheet,
but able to do so in Print Preview. Why can't I see the colours in the
worksheet ( I can see colours of a chart in the worksheet)


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