Hi Willis,
Thank you for your interest in Syncfusion products.
Excel 2003 file format allows only 64 colors available in Excel Palette. All those colors are available via CellStyle.ColorIndex property. However, if you want to write any custom color, they will be indexed, that is, it will be converted to nearest color available in Excel Palette. If you want to use RGB color, you can go for Excel 2007 format. I have placed a sample in the following link:
http://help.syncfusion.com/samples/XlsIO.Silverlight/F94981/F94981.zipPlease try this and let me know if you have any questions.
Regards,
Geetha