<ComboBox DisplayMemberPath="Name" ItemsSource="{Binding Group}" SelectedItem="{Binding EditItem.DictGroup, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}">
     <ComboBox.Style>
           <Style TargetType="{x:Type ComboBox}">
                  <Style.Triggers>
                         <Trigger Property="Validation.HasError" Value="True">
                              <Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}" />
                         </Trigger>
                   </Style.Triggers>
            </Style>
      </ComboBox.Style>
      <ComboBox.ItemContainerStyle>
            <Style TargetType="ComboBoxItem">
                   <Style.Triggers>
                         <DataTrigger Binding="{Binding TriggerHelper}" Value="True">
                                <DataTrigger.Setters>
                                       <Setter Property="Foreground" Value="Tan"/>
                                </DataTrigger.Setters>
                         </DataTrigger>
                    </Style.Triggers>
            </Style>
      </ComboBox.ItemContainerStyle>
</ComboBox>

....
public bool TriggerHelper 
{
      get
      {
            return true;
      }
} 
....