DataGridView单元格颜色改变

我有一个测试程序，构建一个数据表(26行乘6 cols)作为DataGridView的数据源。
数据网格可以通过选择单元格并从3个NumUpDown控件之一输入值来编辑。
每个控件都有特定的颜色。为了保存信息，我将表保存在用户设置中。
我将每个单元格的颜色信息保存在额外的表格单元格中。
问题:当我显示数据网格时，它从表中读取OK值，但是应该根据额外的单元格信息以编程方式添加的单元格颜色没有显示。
(我可以在诊断中看到单元格得到了正确的颜色，但它没有显示在数据网格中。
看起来datagird有一些颜色覆盖(?)。
附件是完整的工作代码。
查看代码中的更多信息。
有没有人可以运行这个并找到为什么颜色没有显示在数据网格中?谢谢。

using System;
using System.Data;
using System.Drawing;
using System.Web.UI.WebControls;
using System.Windows.Forms;
//This is a test program to do the following:
//1. Build an empty data table of 26 rows by 6 columns.
//2. The Table is bind (as DataSource) to a DataGridView.
//3. The DataGrid Colomns 0-3 can be filled with decimals by one of 3 NumericUpDown  controls.
//3. For each cell in cols 0-2 there is one in cols 3-5 that holds info which NumUpDn  was used.
//4. Each NumUpDn has specific color. The corresponding cells have the same backcolor.
//5. When the program starts, it builds the table from the Settings, and displays the  table in the datagrid.
//6. The cells in cols 0-2 are supposed to get the colors ad indicated in cols 3-5, but   they do not.
//To run, make a WinForm with a datagrid 'fareGrid' and 3 NumUpDn controls with background colors.  

namespace DataGridViewTest
{
public partial class Form1 : Form
{
    DataTable fareTable = new DataTable();
    int rowIndex;
    int colIndex;
    public Form1()
    {
        InitializeComponent();
        buildFareTable("fareTable");
        // Load the table (if exists)
        if (Properties.Settings.Default.fareTable != null)
            fareTable = Properties.Settings.Default.fareTable;
        paintFareTableCells(fareTable);
    }
    //Build the initial data table of 26R X 6C
    //Columns 3-5 are used to hold the color information of cols 0-2
    public void buildFareTable(string fareTableName)
    {
        fareTable.TableName = fareTableName;
        fareTable.Columns.Add("Weekday", typeof(Decimal));
        fareTable.Columns.Add("HalfDay", typeof(Decimal));
        fareTable.Columns.Add("Weekend", typeof(Decimal));
        //Adding 3 indexing columns that will hold the tables cell group
        fareTable.Columns.Add("IWeekday", typeof(String));
        fareTable.Columns.Add("IHalfDay", typeof(String));
        fareTable.Columns.Add("Iweekend", typeof(String));
        for (rowIndex = 0; rowIndex < 26; rowIndex++)
        {
            fareTable.Rows.Add();
        }
    }
    public void paintFareTableCells(DataTable fareTable)
    {
        fareDataGrid.DataSource = fareTable;
        for (rowIndex = 0; rowIndex < 26; rowIndex++)
        {
            for (colIndex = 0; colIndex < 3; colIndex++)
            {
                switch (fareTable.Rows[rowIndex][colIndex + 3].ToString())   //Check the color index columns 3-5
                {
                    case "low": fareDataGrid[colIndex, rowIndex].Style.BackColor = Color.Green;  
                        break;
                    case "med": fareDataGrid[colIndex, rowIndex].Style.BackColor = Color.Yellow; 
                        break;
                    case "high": fareDataGrid[colIndex, rowIndex].Style.BackColor = Color.Red;  
                        break;
                    default:
                        break;
                }
                // Diagnostics: Check cell's color. At this point, Colors are OK!  
                Color color = fareDataGrid[colIndex, rowIndex].Style.BackColor;  
            }
        }
    }
   //Update the cells by 3 NumUpDn controls.
    private void numericUpDownFareHigh_ValueChanged(object sender, EventArgs e)
    {
        editFareTableCells(sender);
    }
    private void numericUpDownFareMed_ValueChanged(object sender, EventArgs e)
    {
        editFareTableCells(sender);
    }
    private void numericUpDownFareLow_ValueChanged(object sender, EventArgs e)
    {
        editFareTableCells(sender);
    }
    public void editFareTableCells(Object sender)
    {
        foreach (DataGridViewCell cell in fareDataGrid.SelectedCells)
        {
            if (cell.ColumnIndex < 3)
            {
                cell.Value = ((NumericUpDown)sender).Value;
                cell.Style.BackColor = ((NumericUpDown)sender).BackColor;
                fareDataGrid[cell.ColumnIndex + 3, cell.RowIndex].Value = ((NumericUpDown)sender).Tag;   //Uses cols 3-5 to hold cost level info.
            }
        }
    }
    //Use settings to preserve the data
    private void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
        Properties.Settings.Default.fareTable = this.fareTable;
        Properties.Settings.Default.Save();
    }
}
}