Shape.Fill Property

<Page  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <StackPanel>
    <Ellipse Fill="Red" Width="100" Height="100" />
  </StackPanel>
</Page>

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Shapes;

namespace SDKSample
{
    public partial class SetBackgroundColorOfShapeExample : Page
    {
        public SetBackgroundColorOfShapeExample()
        {
            // Create a StackPanel to contain the shape.
            StackPanel myStackPanel = new StackPanel();

            // Create a red Ellipse.
            Ellipse myEllipse = new Ellipse();

            // Create a SolidColorBrush with a red color to fill the 
            // Ellipse with.
            SolidColorBrush mySolidColorBrush = new SolidColorBrush();

            // Describes the brush's color using RGB values. 
            // Each value has a range of 0-255.
            mySolidColorBrush.Color = Color.FromArgb(255, 255, 255, 0);
            myEllipse.Fill = mySolidColorBrush;
            myEllipse.StrokeThickness = 2;
            myEllipse.Stroke = Brushes.Black;

            // Set the width and height of the Ellipse.
            myEllipse.Width = 200;
            myEllipse.Height = 100;

            // Add the Ellipse to the StackPanel.
            myStackPanel.Children.Add(myEllipse);

            this.Content = myStackPanel;
        }

    }
}