public CIMColor CreateRGBColor( 
   double R,
   double G,
   double B,
   double A
)

/// <summary>
/// Create a mesh symbol that can be applied to a multi-patch feature layer.
/// A mesh symbol is a CIMMeshSymbol object.  Define an array of CIMSymbolLayers which contains a CIMMaterialSymbol layer with the specified properties such as Color, etc.
/// Assign this array of CIMSymbolLayers to the CIMMeshSymbol.
/// ![MeshSymbolOrange](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/mesh-material-orange.png)
/// </summary>
{
  //Note: Run withing QueuedTask
  CIMSymbolLayer[] materialSymbolLayer =
        {
              new CIMMaterialSymbolLayer()
              {
                  Color = ColorFactory.Instance.CreateRGBColor(230,152,0),
                  MaterialMode = MaterialMode.Multiply
              }
         };
  var meshMaterialFillSymbol = new CIMMeshSymbol()
  {
    SymbolLayers = materialSymbolLayer
  };
}

/// <summary>
/// Create a polygon symbol with a dash dot fill. <br/>
/// ![PolygonSymbolDashDot](https://ArcGIS.github.io/arcgis-pro-sdk/images/Symbology/polygon-dash-dot.png)
/// </summary>
{
  //Note: Run withing QueuedTask
  var trans = 50.0;//semi transparent
  CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(0, 0, 0, trans), 2.0, SimpleLineStyle.Solid);

  //Stroke for the fill            
  var dashDot = SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.RedRGB, 1.0, SimpleLineStyle.DashDotDot);
  //Mimic cross hatch
  CIMFill[] solidColorHatch =
        {

           new CIMHatchFill()
          {
              Enable = true,
              Rotation = 0.0,
              Separation = 2.5,
              LineSymbol = new CIMLineSymbol(){SymbolLayers = new CIMSymbolLayer[1] {dashDot } }
          },
           new CIMSolidFill()
          {
              Enable = true,
              Color = ColorFactory.Instance.CreateRGBColor(255, 255, 0)
          },
};
  List<CIMSymbolLayer> symbolLayers = [outline, .. solidColorHatch];
  //This is the polygon symbol with a dash dot fill
  CIMPolygonSymbol dashDotFillPolygon = new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
  //To apply the symbol to a polygon feature layer
  //var renderer = theLayer.GetRenderer() as CIMSimpleRenderer;
  //renderer.Symbol = dashDotFillPolygon.MakeSymbolReference();
  //theLayer.SetRenderer(renderer);
}