SolidColorBrush bgColor;

public ModernBTN() {
  InitializeComponent();
  this.Loaded += delegate (object sender, RoutedEventArgs e) {
    bgColor = (SolidColorBrush)Background;
    MouseEnter += EnterAnim;
    MouseLeave += LeaveAnim;
  };
}

private void EnterAnim(object sender, MouseEventArgs e) {
  DoubleAnimation anim = new DoubleAnimation(0, myBtn.Width, TimeSpan.FromMilliseconds(200));
  indicatorBtn.BeginAnimation(WidthProperty, anim);
  ColorAnimation animC = new ColorAnimation(BGHover, TimeSpan.FromMilliseconds(200));
  myBtn.Background.BeginAnimation(SolidColorBrush.ColorProperty, animC);
}

private void LeaveAnim(object sender, MouseEventArgs e) {          
  DoubleAnimation anim = new DoubleAnimation(myBtn.Width, 0, TimeSpan.FromMilliseconds(200));
  indicatorBtn.BeginAnimation(WidthProperty, anim);
  ColorAnimation animC = new ColorAnimation(Color.FromArgb(bgColor.Color.A, bgColor.Color.R, bgColor.Color.G, bgColor.Color.B), TimeSpan.FromMilliseconds(200));
  myBtn.Background.BeginAnimation(SolidColorBrush.ColorProperty, animC);
}

Por favor, dígame por qué bgColor cambia a BGHover color si pongo los valores de bgColor solo en este fondo .Loaded y = (SolidColorBrush).

ModernBtn.xaml xaml código de mi botón

<UserControl x:Class="ModernButton.ModernBTN"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:ModernButton"
         mc:Ignorable="d" 
         d:DesignHeight="100" d:DesignWidth="200" Name="myBtn" Background = '#FF282829'>
<Grid Width="Auto" Height="Auto" Name="mainGrid">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"></RowDefinition>
        <RowDefinition Height="Auto"></RowDefinition>
    </Grid.RowDefinitions>
    <TextBlock Text="{Binding ElementName=myBtn, Path=BtnText}" FontSize="24" Foreground="#FFC7BBBB" TextAlignment="Center" Grid.Row="0" Name="txtBtn" Padding="{Binding ElementName=myBtn, Path=TextPadding}"></TextBlock>
    <Rectangle Grid.Row="1" Fill="Lime" Height="5" Name="indicatorBtn" Width="0"></Rectangle>
</Grid>