<Window x:Class="WpfApplication1.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1"
        Height="300"
        Width="300"
        x:Name="win">
    <Window.Resources>
        <ResourceDictionary x:Name="comboStyles">
            <Style x:Key="combo1"
                   TargetType="{x:Type ComboBox}">
                <Setter Property="Foreground"
                        Value="Red" />
            </Style>
            <Style x:Key="combo2"
                   TargetType="{x:Type ComboBox}">
                <Setter Property="Foreground"
                        Value="Blue" />
            </Style>
        </ResourceDictionary>
    </Window.Resources>
    <StackPanel>
        <ComboBox Style="{Binding RelativeSource={x:Static RelativeSource.Self},Path=SelectedItem}"
                  ItemsSource="{Binding ElementName=comboStyles,Path=Values}">

        </ComboBox>
    </StackPanel>
</Window>