我无法让TreeViewItem更改行颜色

Question

我有这个树状视图                                                                                                                                                                                                     

                    <TreeView.ItemContainerStyle>
                        <Style TargetType="{x:Type TreeViewItem}"  BasedOn="{StaticResource FilterFieldTreeViewItem}">
                            <EventSetter Event="KeyDown" Handler="OnFilterListItemKeyDown"/>
                        </Style>
                    </TreeView.ItemContainerStyle>
                </TreeView>

并且我无法通过此代码转换TreeViewItem

TreeViewItem ti =（TreeViewItem）_filterDisplay.Items [i]; ti.Foreground = Brushes.Red;

我需要访问特定项目以更改颜色