Colors in XL have to be from the 56 RGB colors in the Color Palette.
You can modify that palette for each workbook (and if you create a
default workbook, for each new book you make) by choosing
Tools/Options/Color, selecting the color to change and clicking
Modify. You can set the color to any color you can display.
However, you can still only have a total of 56 colors available at
any one time.
