public CIMSymbolReference ExclusionSymbol {get; set;}

{
    string colorBrewerSchemesName = "ColorBrewer Schemes (RGB)";
    StyleProjectItem colorBrewerStyle = Project.Current.GetItems<StyleProjectItem>().First(s => s.Name == colorBrewerSchemesName);
    string colorRampName = "Greens (Continuous)";
    //Note: Needs QueuedTask to run
    IList<ColorRampStyleItem> colorRampListFromTheStyle = colorBrewerStyle.SearchColorRamps(colorRampName);

    ColorRampStyleItem colorRampFound = colorRampList[0];
    GraduatedColorsRendererDefinition gcDef = new GraduatedColorsRendererDefinition()
    {
        ClassificationField = "CROP_ACR07",
        ClassificationMethod = ArcGIS.Core.CIM.ClassificationMethod.NaturalBreaks,
        BreakCount = 6,
        ColorRamp = colorRampFound.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
    };
    //Note: Needs QueuedTask to run
    LayerFactory.Instance.CreateLayer<FeatureLayer>(featureLayerCreationParams, MapView.Active.Map);
}