Changing WPF title bar background color

You can also create a borderless window, and make the borders and title bar yourself

Here's an example on how to achieve this:

  <DockPanel HorizontalAlignment="Stretch"
           VerticalAlignment="Stretch"
           LastChildFill="True">

        <Grid DockPanel.Dock="Right"
          HorizontalAlignment="Right">

            <StackPanel Orientation="Horizontal"
                    HorizontalAlignment="Right"
                    VerticalAlignment="Center">

                <Button x:Name="MinimizeButton"
                    KeyboardNavigation.IsTabStop="False"
                    Click="MinimizeWindow"
                    Style="{StaticResource MinimizeButton}" 
                    Template="{StaticResource MinimizeButtonControlTemplate}" />

                <Button x:Name="MaximizeButton"
                    KeyboardNavigation.IsTabStop="False"
                    Click="MaximizeClick"
                    Style="{DynamicResource MaximizeButton}" 
                    Template="{DynamicResource MaximizeButtonControlTemplate}" />

                <Button x:Name="CloseButton"
                    KeyboardNavigation.IsTabStop="False"
                    Command="{Binding ApplicationCommands.Close}"
                    Style="{DynamicResource CloseButton}" 
                    Template="{DynamicResource CloseButtonControlTemplate}"/>

            </StackPanel>
        </Grid>
    </DockPanel>

Handle Click Events in the code-behind.

For MouseDown -

App.Current.MainWindow.DragMove();

For Minimize Button -

App.Current.MainWindow.WindowState = WindowState.Minimized;

For DoubleClick and MaximizeClick

if (App.Current.MainWindow.WindowState == WindowState.Maximized)
{
    App.Current.MainWindow.WindowState = WindowState.Normal;
}
else if (App.Current.MainWindow.WindowState == WindowState.Normal)
{
    App.Current.MainWindow.WindowState = WindowState.Maximized;
}

In WPF the titlebar is part of the non-client area, which can't be modified through the WPF window class. You need to manipulate the Win32 handles (if I remember correctly).
This article could be helpful for you: Custom Window Chrome