Access it by the RGB property of Backcolor.
chart.ChartArea.Fill.BackColor.RGB = RGB(221, 221, 221)
Also if you want hints at how to do something using C# programatically in Excel that you know how to do manually, record a macro. Do it manuually with the macro recording. review the macro code afterwards for how to do the task in VBA. Then you have hints of how to do it in C#.
Cheers!
FillFormat Object (Excel) | Microsoft Docs[
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