<Style TargetType="telerik:RadColorPicker" BasedOn="{StaticResource RadColorPickerStyle}">
	<Setter Property="Padding" Value="5" />
	<Setter Property="SplitButtonStyle">
		<Setter.Value>
			<Style TargetType="telerik:RadSplitButton" BasedOn="{StaticResource RadSplitButtonStyle}">
				<Setter Property="mat:MaterialAssist.FocusBrush"		Value="Transparent"/>
				<Setter Property="mat:MaterialAssist.MouseOverBrush"	Value="Transparent"/>
				<Setter Property="behaviour:ColorPickerButtonSync.IsEnabled" Value="True" />
				<Setter Property="IsChecked" Value="{Binding IsDropDownOpen, RelativeSource={RelativeSource AncestorType=telerik:RadColorPicker}}" />
			</Style>
		</Setter.Value>
	</Setter>
</Style>

<telerik:RadColorPicker Width="70" SelectedColor="Red" IsRecentColorsActive="True">
	<telerik:RadColorPicker.ContentTemplate>
		<DataTemplate>
			<Rectangle	Width="35" Height="25"
									Fill="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type telerik:RadColorPicker}},
									Path=SelectedColor, Converter={StaticResource mediaToBrushConverter}}" />
		</DataTemplate>
	</telerik:RadColorPicker.ContentTemplate>

	<telerik:RadColorPicker.AdditionalContent>
		<ContentControl HorizontalContentAlignment="Stretch" Content="{Binding}" >
			<ContentControl.ContentTemplate>
				<DataTemplate>
					<Grid>
						<telerik:RadButton	Height="26" BorderThickness="0"
														HorizontalContentAlignment="Left"
														Command="{Binding OnClickOpenEditColorsCommand}"
														CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type telerik:RadColorPicker}}}"
														>
							<telerik:RadButton.Content>
								<TextBlock Text="More Colors..." Margin="26 0 0 0" />
							</telerik:RadButton.Content>
						</telerik:RadButton>
					</Grid>
				</DataTemplate>
			</ContentControl.ContentTemplate>
		</ContentControl>
	</telerik:RadColorPicker.AdditionalContent>
</telerik:RadColorPicker>

public class ColorPickerButtonSync
    {
        public static bool GetIsEnabled(DependencyObject obj)
        {
            return (bool)obj.GetValue(IsEnabledProperty);
        }

        public static void SetIsEnabled(DependencyObject obj, bool value)
        {
            obj.SetValue(IsEnabledProperty, value);
        }

        public static readonly DependencyProperty IsEnabledProperty =
            DependencyProperty.RegisterAttached("IsEnabled", typeof(bool), typeof(ColorPickerButtonSync), new PropertyMetadata(false, new PropertyChangedCallback(OnIsEnabledChanged)));

        private static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var button = d as RadSplitButton;
            button.IsToggle = true;
            button.Loaded += Button_Loaded;
            button.Checked += SplitButton_Checked;
            button.Unchecked += SplitButton_Unchecked;
            button.DropDownOpened += Button_DropDownOpened;
            button.DropDownClosed += Button_DropDownClosed;
        }

        private static void Button_Loaded(object sender, RoutedEventArgs e)
        {
            var splitButton = sender as RadSplitButton;
            var colorSelector = splitButton.DropDownContent as RadColorSelector;
            colorSelector.Loaded += (s, a) =>
            {
                var rootElement = colorSelector.ChildrenOfType<Grid>().First(x => x.Name == "RootElement");
                Grid.SetRow(rootElement.Children[1], 4);
                Grid.SetRow(rootElement.Children[2], 5);
                Grid.SetRow(rootElement.Children[3], 6);
                Grid.SetRow(rootElement.Children[4], 1);
                Grid.SetRow(rootElement.Children[5], 2);
                Grid.SetRow(rootElement.Children[6], 3);
            };

            var dropDownPart = splitButton.ChildrenOfType<RadToggleButton>().First(x => x.Name == "DropDownPart");
            var rectangle = splitButton.ChildrenOfType<Rectangle>().First(x => x.Name == "Separator");
            //System.InvalidOperationException: 'Sequence contains no matching element'
            dropDownPart.Visibility = Visibility.Collapsed;
            rectangle.Visibility = Visibility.Collapsed;
        }

        private static void Button_DropDownClosed(object sender, RoutedEventArgs e)
        {
            var splitButton = sender as RadSplitButton;
            splitButton.IsChecked = false;
        }

        private static void Button_DropDownOpened(object sender, RoutedEventArgs e)
        {
            var splitButton = sender as RadSplitButton;
            splitButton.IsChecked = true;
        }

        private static void SplitButton_Checked(object sender, RoutedEventArgs e)
        {
            var splitButton = sender as RadSplitButton;
            var colorPicker = splitButton.ParentOfType<RadColorPicker>();
            colorPicker.IsDropDownOpen = true;
        }

        private static void SplitButton_Unchecked(object sender, RoutedEventArgs e)
        {
            var splitButton = sender as RadSplitButton;
            var colorPicker = splitButton.ParentOfType<RadColorPicker>();
            colorPicker.IsDropDownOpen = false;
        }
    }