How to bind a button icon to the selected item of a drop-down list?

Question 1

On the page 17 of the walkthrough you have an example of what you are trying to achieve.

You can download it here : http://fluent.codeplex.com/documentation

Taken from the walkthrough :

enter image description here

<fluent1:Ribbon>
    <fluent1:Ribbon.Menu>
        <fluent1:Backstage />
    </fluent1:Ribbon.Menu>
    <fluent1:RibbonTabItem Header="Home">
        <fluent1:RibbonGroupBox Header="Clipboard">
            <!--  The following code shows standard mode for color gallery  -->
            <fluent1:DropDownButton Header="Standard">
                <!--  It's possible to create custom icon to present selected color  -->
                <fluent1:DropDownButton.Icon>
                    <Grid Width="16" Height="16">
                        <Image Source="Images\FontColor.png" />
                        <Border Height="4"
                                VerticalAlignment="Bottom"
                                BorderThickness="0">
                            <Border.Background>
                                <SolidColorBrush
                                    Color="{Binding ElementName=ColorGalleryStandard, Path=SelectedColor, FallbackValue=Black}" />
                            </Border.Background>
                        </Border>
                    </Grid>
                </fluent1:DropDownButton.Icon>
                <fluent1:ColorGallery x:Name="ColorGalleryStandard"
                                        IsNoColorButtonVisible="False"
                                        SelectedColor="Red" />
                <fluent1:MenuItem Header="A Menu Item" Icon="Images\Pink.png" />
            </fluent1:DropDownButton>
        </fluent1:RibbonGroupBox>
    </fluent1:RibbonTabItem>
</fluent1:Ribbon>

UPDATE

I see nothing wrong in your code, I've pasted it and it ran successfully, here is it again pasted from my working test.

<Window x:Class="WpfApplication8.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:fluent1="clr-namespace:Fluent;assembly=Fluent"
        xmlns:system="clr-namespace:System;assembly=mscorlib"
        Title="MainWindow"
        Width="525"
        Height="350">
    <Window.Resources>
        <ObjectDataProvider x:Key="colorsTypeOdp"
                            MethodName="GetType"
                            ObjectType="{x:Type system:Type}">
            <ObjectDataProvider.MethodParameters>
                <system:String>
                    System.Windows.Media.Colors, PresentationCore,
                    Version=3.0.0.0, Culture=neutral,
                    PublicKeyToken=31bf3856ad364e35
                </system:String>
            </ObjectDataProvider.MethodParameters>
        </ObjectDataProvider>

        <ObjectDataProvider x:Key="colorPropertiesOdp"
                            MethodName="GetProperties"
                            ObjectInstance="{StaticResource colorsTypeOdp}" />
    </Window.Resources>
    <fluent1:DropDownButton Name="btnCommentColor" Header="Comments">
        <fluent1:DropDownButton.LargeIcon>
            <Grid Width="32" Height="32">
                <Image Source="Icons\BlueLarge.png" />
                <Border Height="32"
                        VerticalAlignment="Bottom"
                        BorderThickness="0"
                        CornerRadius="2">
                    <Border.Background>
                        <SolidColorBrush
                            Color="{Binding ElementName=galCommentColor, Path=SelectedValue, FallbackValue=Green}" />
                    </Border.Background>
                </Border>
            </Grid>
        </fluent1:DropDownButton.LargeIcon>
        <fluent1:Gallery Name="galCommentColor"
                         ItemsSource="{Binding Source={StaticResource colorPropertiesOdp}}"
                         MaxItemsInRow="12"
                         SelectedValuePath="Name">
            <fluent1:Gallery.ItemTemplate>
                <DataTemplate>
                    <Border Width="25"
                            Height="25"
                            VerticalAlignment="Stretch"
                            Background="{Binding Name}"
                            BorderBrush="Black"
                            BorderThickness="1"
                            CornerRadius="2"
                            ToolTip="{Binding Path=Name}" />
                </DataTemplate>
            </fluent1:Gallery.ItemTemplate>
        </fluent1:Gallery>
    </fluent1:DropDownButton>

</Window>

Question 2

Thanks to Aybe for confirming that it's not me. I did get the result I wanted by using a converter on the LargeIcon property of the DropDownButton.

Here's the XAML:

<Fluent:DropDownButton Name="btnCommentColor" Header="Comments" HasTriangle="False" LargeIcon="{Binding ElementName=galCommentColor, Path=SelectedValue, Converter={StaticResource ColorNameToBorderConverter_Key}}">
    <Fluent:Gallery x:Name="galCommentColor" ItemsSource="{Binding Source={StaticResource colorPropertiesOdp}}" SelectedValuePath="Name" MaxItemsInRow="12" SelectedIndex="51">
        <Fluent:Gallery.ItemTemplate>
            <DataTemplate>
                <Border ToolTip="{Binding Name}" BorderThickness="1" CornerRadius="2" BorderBrush="Black" Width="25" Height="25" VerticalAlignment="Stretch" Background="{Binding Name}" /> 
            </DataTemplate>
        </Fluent:Gallery.ItemTemplate>
    </Fluent:Gallery>
</Fluent:DropDownButton>

And the code:

Public Class ColorNameToBorderConverter
    Implements IValueConverter

    Public Function Convert(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.Convert
        If ApplicationIsInDesignMode Then value = "Black"

        If TypeOf value Is String Then
            Return New Border With {
                .Height = 32,
                .BorderThickness = New Thickness(1),
                .BorderBrush = New SolidColorBrush(System.Windows.Media.Colors.Black),
                .CornerRadius = New CornerRadius(2, 2, 2, 2),
                .VerticalAlignment = VerticalAlignment.Bottom,
                .Background = New SolidColorBrush(ColorConverter.ConvertFromString(value))
            }
        Else
            Throw New InvalidOperationException("Unsupported type [" & value.GetType.ToString & "]")
        End If
    End Function

    Public Function ConvertBack(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.ConvertBack
        Throw New NotImplementedException
    End Function

    Private Shared ReadOnly Property ApplicationIsInDesignMode() As Boolean
        Get
            Return CBool(DesignerProperties.IsInDesignModeProperty.GetMetadata(GetType(DependencyObject)).DefaultValue)
        End Get
    End Property
End Class