XL can only display the 56 colors in the Color Palette. Choose
Tools/Options/Color. Select a color you no longer want, and click
Modify. Choose the color you want from the popup.
This choice applies at the workbook level. To make it apply to all new
workbooks, you'll need to create/modify a default workbook. See Help for
details.
You can also import, in the Color Options dialog, palettes from other
workbooks.