C # excel cell multicolor text

I wrote an app in C # that writes some data (strings) to excel-document. I need to write multiple lines in one cell. After that I try to color these lines. In my example, there are four different lines that need to be colored differently in the same 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 to do it like in the example, always the first and last are correct, but all others are false. For this project I am using Windows 7 Professional, Microsoft Excel 2010, and 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();
            }
        }
    }
}