WPF modified combo box selection not working

I am trying to create a color picker which is based on combo box control. I am able to attain the desired look for the combo box ,but the combo box selection is not working. Below is the XAML code for my combo box and templates .I have overriden the default style of the combo box as, i need my combo box to appear as a circular button.

<Window x:Class="WpfApp5.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"
    mc:Ignorable="d">
<Window.Resources>

    <Style x:Key="ComboBoxButtonStyle" TargetType="{x:Type ToggleButton}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ToggleButton}">
                    <Border 
                    Background="White" 
                    x:Name="border" 
                    CornerRadius="30" 
                    BorderThickness="1,1,1,1"
                    BorderBrush="Black" >
                        <ContentPresenter  />
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    
    <Style  x:Key="RoundComboBox" TargetType="{x:Type ComboBox}">
        <Setter Property="HorizontalContentAlignment" Value="Center"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
        <Setter Property="Cursor" Value="Hand"/>
        <Setter Property="ItemContainerStyle">
            <Setter.Value>
                <Style TargetType="ComboBoxItem">
                    <Style.Setters>
                        <Setter Property="Template">
                            <Setter.Value>
                                <ControlTemplate>
                                    <Button Name="btn" Background ="{TemplateBinding Property=Background}">
                                        <Button.Template>
                                            <ControlTemplate TargetType="Button">
                                                <Ellipse Width="20" Height="20" Fill="{Binding ElementName=btn,Path=Background}" />
                                            </ControlTemplate>
                                        </Button.Template>
                                    </Button>
                                </ControlTemplate>
                            </Setter.Value>
                        </Setter>
                    </Style.Setters>
                </Style>
            </Setter.Value>
        </Setter>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type ComboBox}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition/>
                        </Grid.ColumnDefinitions>
                        <ToggleButton Grid.Column="0" Margin="2"
                        Height="{TemplateBinding Height}"
                                      HorizontalAlignment="Stretch"
                                      VerticalAlignment="Stretch"
                        Style="{StaticResource ComboBoxButtonStyle}"
                        Focusable="True"      
                        IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                        ClickMode="Press">
                        </ToggleButton>
                        <ContentPresenter />
                        <Popup Name="Popup"
                        Placement="RelativePoint"
                        IsOpen="{TemplateBinding IsDropDownOpen}"
                        AllowsTransparency="True" 
                        Focusable="False"
                        PopupAnimation="Fade">
                            <Grid Name="DropDown"
                            SnapsToDevicePixels="True"                
                            MinWidth="{TemplateBinding ActualWidth}"
                            MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                <Border 
                                x:Name="DropDownBorder"
                                BorderThickness="1"
                                CornerRadius="15"
                                Background="Azure"
                                />
                                <ScrollViewer  SnapsToDevicePixels="True">
                                    <StackPanel Orientation="Vertical" IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
                                </ScrollViewer>
                            </Grid>
                        </Popup>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="20"/>
        <RowDefinition Height="30"/>
        <RowDefinition Height="30"/>
    </Grid.RowDefinitions>
    <ComboBox Grid.Row="2" Width="30" Style="{StaticResource RoundComboBox}"  >
        <ComboBoxItem Height="30" Background="Green"  />
        <ComboBoxItem Background="Blue"/>
        <ComboBoxItem Height="30" Background="Red" />
        <ComboBoxItem Background="Orange"/>
    </ComboBox >

</Grid>