//XAML code
<Grid x:Name="btn_2" MouseLeftButtonDown="MouseLeftButtonDown_Button">
<Grid.Background>
<SolidColorBrush />
</Grid.Background>
<TextBlock x:Name="btn_2_tb" Text="2" TextAlignment="Center" Foreground="Black"></TextBlock>
</Grid>
//C# code
ColorAnimation ca = new ColorAnimation();
ca.Duration = new Duration(TimeSpan.FromSeconds(1));
ca.From = Color.FromArgb(255, 255, 0, 0);
ca.To = Color.FromArgb(255, 0, 0, 255);
Storyboard.SetTarget(ca, btn_0 );
Storyboard.SetTargetProperty(ca, new PropertyPath("UIElement.Background.Color"));
// alternative tested PropertyPath
// "Panel.Background.Color"
// "Panel.Background.SolidColorBrush.Color"
// "(Panel.Background).(SolidColorBrush.Color)"
Storyboard sb = new Storyboard();
sb.Children.Add(ca);
sb.Begin();