The XML format you're using comes from Excel 2003, which was limited to a palette of 56 colours. Any colours not in the palette will be mapped to the nearest match.
You can see the palette here:
Color Palette and the 56 Excel ColorIndex Colors
I haven't seen any way to override the palette with this file format. If you need custom colours, then you'll probably need to use The OpenXML SDK
] to generate your spreadsheet instead.