<Canvas xmlns="http://schemas.microsoft.com/client/2007"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Canvas.Triggers>
    <EventTrigger RoutedEvent="Canvas.Loaded">
      <BeginStoryboard>
        <Storyboard>
          <ColorAnimation Storyboard.TargetName="circleBrush"
                          Storyboard.TargetProperty="Color"
                          From="Green" To="Red" Duration="0:0:0.5"
                          AutoReverse="True" RepeatBehavior="Forever"/>
        </Storyboard>
      </BeginStoryboard>
    </EventTrigger>
  </Canvas.Triggers>
  <Ellipse Width="200" Height="200">
    <Ellipse.Fill>
      <SolidColorBrush x:Name="circleBrush" Color="Blue" />
    </Ellipse.Fill>
  </Ellipse>
</Canvas>