How do I multicolor a text in an Excelcell with C#

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C#

MS-Excel

I have written an application in c# which writes some data (strings) into an excel-document. I need to write several strings into one cell. After this I try to color these strings. In my example there are four different strings that need to be colored differently in one cell. The first one should be blue, the second one should be green, the third one should be black and the last one should be red. When I try it as in the example, always the first and the last one are correct, but all the others are false. For this project I am using Windows 7 Professional, Microsoft Excel 2010, Visual Studio 2012 Ultimate. How can I fix my source?

This is my Source:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;
using System.Reflection;
using System.Windows;
using System.Runtime.InteropServices;
using Microsoft.Office.Interop.Excel;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        private Microsoft.Office.Interop.Excel.Application excel = null;
        private Microsoft.Office.Interop.Excel.Workbook workbook = null;
        public Microsoft.Office.Interop.Excel.Worksheet _sheet = null;
        public Form1()
        {
            myMethod();
        }

        private void myMethod()
        {
            excel = new Microsoft.Office.Interop.Excel.ApplicationClass();

            object missing = Missing.Value;
            excel.Workbooks.Add(missing);
            workbook = excel.ActiveWorkbook;
            workbook.Worksheets.Add(missing, missing, missing);

            _sheet =
                    (Microsoft.Office.Interop.Excel.Worksheet)workbook.Worksheets[1];
            _sheet.Name = "Sheet";

            string[] part = { "exampleBluee", "exampleGreen", "exampleBlack", "exampleReddd" };
            string[] faults = { "f1", "f2", "f3", "f4" };
            _sheet.Columns.WrapText = true;

            Microsoft.Office.Interop.Excel.Range position = _sheet.get_Range("A1");
            position.VerticalAlignment = Microsoft.Office.Interop.Excel.Constants.xlCenter;
            for (int i = 0; i < 4; i++)
            {
                position.Value2 += part[i] + " ";
                int fehler = i;

                // default color
                var myColor = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.Pink);

                if (faults[fehler] == "f1")
                {
                    myColor = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.RoyalBlue);
                }
                else if (faults[fehler] == "f2")
                {
                    myColor = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.Green);
                }
                else if (faults[fehler] == "f3")
                {
                    myColor = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.Black);
                }
                else if (faults[fehler] == "f4")
                {
                    myColor = System.Drawing.ColorTranslator.ToOle(System.Drawing.Color.Red);
                }

                var nLenTotal = position.Value2.ToString().Length;
                int lenTxt = part[i].Length;
                // expected colorized text. (multicolor text in one cell)
                position.Characters[nLenTotal - lenTxt, lenTxt].Font.Color = myColor;
            }
            ((Microsoft.Office.Interop.Excel.Range)_sheet.Columns[1]).EntireRow.ColumnWidth = 15;

            saveExcel();
        }
        public void saveExcel()
        {
            string appPath = System.IO.Path.GetDirectoryName(
                Assembly.GetEntryAssembly().Location);
            string filename = Path.Combine(appPath, "MyDocument.xlsx");

            try
            {
                workbook.SaveAs(filename);
                workbook.Close();
            }
            catch (Exception)
            {
                workbook.Close();
            }
        }
    }
}