Excel versions up to 2003 have only a palette of 54 colors that you can use. Internally, those colors are stores as an index to a color palette, and most properties in FlexCel return this index. 

Using this property, you can map that index to a real color like clBlue or clRed. 

Note that while this limit of 54 colors was removed in Excel 2007, you can still use color indexes instead of real colors in 2007. 

For an in-depth explanation of the Excel color palette, see Using the API documentation. 

TFlexCelImport Properties