public string ExclusionClause {get; set;}

string colorBrewerSchemesName = "ColorBrewer Schemes (RGB)";
StyleProjectItem style = Project.Current.GetItems<StyleProjectItem>().First(s => s.Name == colorBrewerSchemesName);
string colorRampName = "Greens (Continuous)";
IList<ColorRampStyleItem> colorRampList = await QueuedTask.Run(() =>
{
  return style.SearchColorRamps(colorRampName);
});
ColorRampStyleItem colorRamp = colorRampList[0];

await QueuedTask.Run(() =>
{
  GraduatedColorsRendererDefinition gcDef = new GraduatedColorsRendererDefinition()
  {
    ClassificationField = "CROP_ACR07",
    ClassificationMethod = ArcGIS.Core.CIM.ClassificationMethod.NaturalBreaks,
    BreakCount = 6,
    ColorRamp = colorRamp.ColorRamp,
    SymbolTemplate = SymbolFactory.Instance.ConstructPolygonSymbol(
                            ColorFactory.Instance.GreenRGB, SimpleFillStyle.Solid, null).MakeSymbolReference(),
    ExclusionClause = "CROP_ACR07 = -99",
    ExclusionSymbol = SymbolFactory.Instance.ConstructPolygonSymbol(
                            ColorFactory.Instance.RedRGB, SimpleFillStyle.Solid, null).MakeSymbolReference(),
    ExclusionLabel = "No yield",
  };
  var featureLayerCreationParams = new FeatureLayerCreationParams((new Uri(@"c:\Data\CountyData.gdb\Counties")))
  {
    Name = "Crop",
    RendererDefinition = gcDef
  };
  LayerFactory.Instance.CreateLayer<FeatureLayer>(featureLayerCreationParams, MapView.Active.Map);
});