public ImageSource PreviewImage {get;}

//Note: call within QueuedTask.Run()
CIMSymbol symbol = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.GreenRGB, 1.0, SimpleMarkerStyle.Circle);
  //You can generate a swatch for a text symbols also.
  var si = new SymbolStyleItem()
  {
      Symbol = symbol,
      PatchHeight = 64,
      PatchWidth = 64
  };
  return si.PreviewImage;