public ImageSource PreviewImage {get;}

{
    //Note: call within QueuedTask.Run()
    CIMSymbol symbolForSwatch = 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 = symbolForSwatch,
        PatchHeight = 64,
        PatchWidth = 64
    };
}