How to set Foreground color for Watermark text in SfTextBoxExt?

<Input:SfTextBoxExt Name="sftextbox"  AutoCompleteMode="Suggest"  
   AutoCompleteSource="{Binding AutoCompleteSource, Mode=TwoWay}">
     <Input:SfTextBoxExt.WatermarkTemplate>
       <DataTemplate>
            <TextBlock Text="Type S to Start" Foreground="Green"/>
       </DataTemplate>
     </Input:SfTextBoxExt.WatermarkTemplate>
</Input:SfTextBoxExt>

public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
            var str = "<DataTemplate xmlns=\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\">" +
                 "<TextBlock Text=\"Type S to Start\" Foreground=\"Green\" />" +
            "</DataTemplate>";
            DataTemplate template = (DataTemplate)Windows.UI.Xaml.Markup.XamlReader.Load(str);
            sftextbox.WatermarkTemplate = template;
           Collection c = new Collection();
            this.DataContext = c;            
        }
   public class Collection
        {
            public List<string> AutoCompleteSource
            {
                get
                {
                    return new List<string>() { "Sam", "Sarvan", "Stephen" };
                }
            }
        }
    }