Yes, you can. Go to Tools > Options and select the Color tab. Select any
color you like and Modify it to your taste (there's a color wheel available
with decent colors and excellent shades of gray, or you can specify your own
RGB color). Just leave black and white alone, but the rest are free game.
Incidentally, I have our company colors pre-loaded in my Personal.xls
workbook (Window > Unhide) so that when I open or create a spreadsheet and
need our colors, I don't have to re-create the colors every time. I just
unhide Personal.xls, go back to the sheet I was working on, go to Tools >
Options > Color, and in the "Copy colors from" drop-down box select
Personal.xls. Then I hit OK and hide my Personal.xls workbook again.