Add a RadColorBox on the form and use the following code:
this.radColorBox1.Value = Color.Black;
RadColorSelector selector = this.radColorBox1.ColorDialog.ColorDialogForm.RadColorSelector as RadColorSelector;
selector.OldColor = Color.Green;
Workaround:
this.radColorBox1.ColorDialog.ColorDialogForm.SelectedColor = Color.Green;
this.radColorBox1.Value = Color.Black;