WPF MultiBinding

<Window x:Class="WpfApp84.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:system="clr-namespace:System;assembly=mscorlib"
        xmlns:local="clr-namespace:WpfApp84"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:RGBConverter x:Key="rgbConverter"/>
    </Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Rectangle Stroke="Black" StrokeThickness="3" Margin="4">
            <Rectangle.Fill>
                <MultiBinding Converter="{StaticResource rgbConverter}">
                    <Binding Path="Value" ElementName="_red"/>
                    <Binding Path="Value" ElementName="_green"/>
                    <Binding Path="Value" ElementName="_blue"/>
                </MultiBinding>
            </Rectangle.Fill>
        </Rectangle>
        <Slider  Minimum="0" Maximum="255" Margin="4" x:Name="_red" Grid.Row="1"                   
                 TickFrequency="1" IsSnapToTickEnabled="True" />
        <Slider Minimum="0" Maximum="255" Margin="4" x:Name="_green" Grid.Row="2"  
                TickFrequency="1" IsSnapToTickEnabled="True" />
        <Slider Minimum="0" Maximum="255" Margin="4" x:Name="_blue" Grid.Row="3" 
                TickFrequency="1" IsSnapToTickEnabled="True" />
        <Grid Grid.Row="4">
            <Grid.RowDefinitions>
                <RowDefinition/>
                <RowDefinition/>
                <RowDefinition/>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <StackPanel Grid.Row="0" Orientation="Horizontal">
                <TextBlock Grid.Column="0" Text="Red:"/>
                <TextBlock Grid.Column="1" Text="{Binding Path=Value,ElementName=_red}" HorizontalAlignment="Left"/>
            </StackPanel>
            <StackPanel Grid.Row="1" Orientation="Horizontal">
                <TextBlock Grid.Column="0" Text="Green:"/>
                <TextBlock Grid.Column="1" Text="{Binding Path=Value,ElementName=_green}"/>
            </StackPanel>
            <StackPanel Grid.Row="2" Orientation="Horizontal">
                <TextBlock Grid.Column="0" Text="Blue:"/>
                <TextBlock Grid.Column="1" Text="{Binding Path=Value,ElementName=_blue}"/>
            </StackPanel>
        </Grid>
    </Grid>
</Window>


using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp84
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class RGBConverter : IMultiValueConverter
    {
        SolidColorBrush brush = new SolidColorBrush();
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            brush.Color = Color.FromRgb(System.Convert.ToByte(values[0]),
                System.Convert.ToByte(values[1]),
                System.Convert.ToByte(values[2]));
            return brush;
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}