Hi Mark,
Thank you very much for using syncfusion products.
We don't provide support to copy custom colors set in the workbook through CopyAll flags. The following code snippet will help you to copy the custom colors to destination workbook.
Code Snippet(C#):
dWorkbook.Worksheets.AddCopy(sWorkbook.Worksheets[0], ExcelWorksheetCopyFlags.CopyAll|ExcelWorksheetCopyFlags.CopyPalette);
I have also attached the work around sample with this update. Please try this at your side and let us know if this helps you.
Please let us know if you have any clarifications.
Thanks,
Jawahar.
CopySheet_613b0fe2.zip