public CIMColor SelectionColor {get; set;}

var featureLayer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // get the CIM definition of the layer
  var layerDef = featureLayer.GetDefinition() as ArcGIS.Core.CIM.CIMBasicFeatureLayer;
  // disable the default symbol
  layerDef.UseSelectionSymbol = false;
  // assign a new color
  layerDef.SelectionColor = ColorFactory.Instance.RedRGB;
  // apply the definition to the layer
  featureLayer.SetDefinition(layerDef);

  if (!featureLayer.IsVisible)
    featureLayer.SetVisibility(true);
  //Do a selection

  MapView.Active.SelectFeatures(MapView.Active.Extent);
});