为什么ColorAnimation不能使用TargetProperty="Fill"？

<Window x:Class="ColorAnimationExample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Color Animation Example" Height="450" Width="800">
    <Window.Resources>
        <Storyboard x:Key="ColorAnimationStoryboard">
            <ColorAnimation From="Red" To="Blue" Duration="0:0:2" RepeatBehavior="Forever" AutoReverse="True"
                            Storyboard.TargetName="myRectangle" Storyboard.TargetProperty="(Rectangle.Fill).(SolidColorBrush.Color)" />
        </Storyboard>
    </Window.Resources>
    <Grid>
        <Rectangle x:Name="myRectangle" Width="100" Height="100" Fill="Red" />
        <Button Content="Start Animation" Click="Button_Click" HorizontalAlignment="Center" VerticalAlignment="Bottom" Margin="0,0,0,20" />
    </Grid>
</Window>

using System.Windows;

namespace ColorAnimationExample
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Storyboard storyboard = (Storyboard)FindResource("ColorAnimationStoryboard");
            storyboard.Begin();
        }
    }
}