format character in excel cell and preserve old formatting

excel_interop_character.cs

using Microsoft.Office.Interop.Excel;
using System;
using System.Collections.Generic;
using System.Drawing;

namespace excelinterop
{
    class Program
    {

        static void Main(string[] args)
        {
            var app = new Application();
            app.Visible = true;
            var myBook = app.Workbooks.Open(@"c:\temp\test.xlsx");
            var mySheet = (Worksheet)myBook.Worksheets[1];
            Console.WriteLine("sheet " + mySheet.Name);
            var cell = mySheet.Cells[1, 1] as Microsoft.Office.Interop.Excel.Range;
            Console.WriteLine("cell " + cell.Value);

            var colors = ReadColor(cell);
            AddText(cell, colors, "abc", Color.DarkRed);
        }

        private static void AddText(Microsoft.Office.Interop.Excel.Range cell, List<double> colors, string v, Color color)
        {
            cell.Value2 += v;
            for (var x = 1; x <= colors.Count; x++)
            {
                cell.Characters[x, 1].Font.Color = colors[x - 1];
            }

            cell.Characters[colors.Count + 1, v.Length].Font.Color = color;
        }

        private static List<double> ReadColor(Microsoft.Office.Interop.Excel.Range cell)
        {
            var colors = new List<double>();

            for (var x = 1; x <= cell.Characters.Count; x++)
            {
                var c = cell.Characters[x, 1];
                colors.Add((double)c.Font.Color);
            }
            return colors;
        }


    }
}