According to this thread: https://www.telerik.com/forums/how-to-force-series-color
Children collection should be used to set specific color for pie element
radChartView1.Series.Children.BackColor = Color.Red;
However, I noticed "strange" behavior in data bound mode: As soon as reference to Children is made labels are not shown.
You can see this behavior in sample below:
Uncomment any of 2 lines below and you can see labels are not there
Simple form with load event:
And data object: