Text and object colors are stored internally as indexes into a palette. Every workbook is created with a default color palette. Use the IColors interface to query the color palette or to modify it.

Most objects provide the option of specifying a color as an index into this color palette or as an instance of SpreadsheetGear.Drawing.Color, in which case the actual color to be used will be the color in the palette which is closest to the specified SpreadsheetGear.Drawing.Color.

Workbooks with a custom color palette are saved with the custom palette.

Reference