積層棒グラフ（積み上げ形式の棒グラフ）をマウスでクリックした時、選択された部分の色を変更するには、選択要素のスタイル（SelectionStyle）のFillColorプロパティを使用します。
なお、StrokeColorプロパティを用いると、選択部分の外枠の色を設定することができます。

以下に、Renderingイベントを利用して選択時の色を変更するサンプルを記載します。


◎サンプルコード（VB）

Imports C1.Chart
Imports C1.Win.Chart

Public Class Form1
    Dim fillColors As Color() = New Color() {Color.DarkBlue, Color.DarkRed, Color.DarkGreen}
    Dim strokeColors As Color() = New Color() {Color.LightBlue, Color.LightPink, Color.LightGreen}

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        FlexChart1.Series.Clear()

        ' 積層棒グラフのデータ系列の作成
        FlexChart1.BeginUpdate()
        Dim item1 As Series = New Series() With
        {
            .Binding = "Y",
            .BindingX = "X",
            .DataSource = New PointF() {New PointF(1, 1), New PointF(2, 2), New PointF(3, 3)}
        }
        FlexChart1.Series.Add(item1)
        Dim item2 As Series = New Series() With
        {
            .Binding = "Y",
            .BindingX = "X",
            .DataSource = New PointF() {New PointF(1, 1), New PointF(2, 1), New PointF(3, 1)}
        }
        FlexChart1.Series.Add(item2)
        Dim item3 As Series = New Series() With
        {
            .Binding = "Y",
            .BindingX = "X",
            .DataSource = New PointF() {New PointF(1, 2), New PointF(2, 2), New PointF(3, 2)}
        }
        FlexChart1.Series.Add(item3)

        ' チャートの設定
        FlexChart1.ChartType = ChartType.Column
        FlexChart1.Stacking = Stacking.Stacked
        ' 選択モードの設定
        FlexChart1.SelectionMode = ChartSelectionMode.Point 'ポイント選択

        FlexChart1.EndUpdate()
    End Sub

    Private Sub FlexChart1_Rendering(sender As Object, e As RenderEventArgs) Handles FlexChart1.Rendering
        If FlexChart1.SelectedSeries IsNot Nothing Then
            For i As Integer = 0 To FlexChart1.Series.Count - 1
                If FlexChart1.SelectedSeries Is FlexChart1.Series(i) Then
                    FlexChart1.SelectionStyle.FillColor = fillColors(i)
                    FlexChart1.SelectionStyle.StrokeColor = strokeColors(i)
                End If
            Next
        End If
    End Sub
End Class

◎サンプルコード（C#）

using C1.Chart;
using C1.Win.Chart;

namespace prj_FlexChart
{
    public partial class Form1 : Form
    {
        Color[] fillColors = new Color[] { Color.DarkBlue, Color.DarkRed, Color.DarkGreen };
        Color[] strokeColors = new Color[] { Color.LightBlue, Color.LightPink, Color.LightGreen };
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            flexChart1.Series.Clear();

            // 積層棒グラフのデータ系列の作成
            flexChart1.BeginUpdate();
            Series item1 = new Series() {
                Binding="Y",
                BindingX="X",
                DataSource=new PointF[] {new PointF(1,1), new PointF(2, 2) , new PointF(3, 3) }
            };
            flexChart1.Series.Add(item1);
            Series item2 = new Series()
            {
                Binding = "Y",
                BindingX = "X",
                DataSource = new PointF[] { new PointF(1, 1), new PointF(2, 1), new PointF(3, 1) }
            };
            flexChart1.Series.Add(item2);
            Series item3 = new Series()
            {
                Binding = "Y",
                BindingX = "X",
                DataSource = new PointF[] { new PointF(1, 2), new PointF(2, 2), new PointF(3, 2) }
            };
            flexChart1.Series.Add(item3);

            // チャートの設定
            flexChart1.ChartType = ChartType.Column;
            flexChart1.Stacking = Stacking.Stacked;
            // 選択モードの設定
            flexChart1.SelectionMode = ChartSelectionMode.Point; //ポイント選択

            flexChart1.EndUpdate();
        }

        private void flexChart1_Rendering(object sender, RenderEventArgs e)
        {
            if (flexChart1.SelectedSeries != null)
            {
                for (int i = 0;i < flexChart1.Series.Count; i++)
                {
                    if (flexChart1.SelectedSeries == flexChart1.Series[i])
                    {
                        flexChart1.SelectionStyle.FillColor = fillColors[i];
                        flexChart1.SelectionStyle.StrokeColor = strokeColors[i];
                    }
                }
            }
        }
    }
}