Both approaches should be working fine. Please, try setting the ChartSeriesItems' FillType:
radChart1.Series.Items.Appearance.FillStyle.FillType = FillType.Solid;
e.SeriesItem.Appearance.FillStyle.FillType = FillType.Solid;
If you still have problems setting the color, please, send a simple project that we can analyze.
the Telerik team
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