Custom color does not gets applied in the exported file (no matter if you export to .xslx or image file like .png ). The problem occurs if Alpha Transparency of the color is specified. 

The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

Workaround:
Set the Background of the RadCartesianChart to White.