public bool UseSelectionSymbol {get; set;}

{
    // get the CIM definition of the layer
    // Note: needs to be called on the MCT
    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);
}