Preferences/Color/Modify
Note that XL can only display the 56 colors of the palette - if you
change a color that already is being shown on the sheet, that color will
change.
Note also that the color palette is workbook-specific. To change your
default palette, create a default workbook template (named "Workbook"-
no quotes, no extension) with your desired colors and save it in the XL
Startup folder.