利用 GetPixel 及 SetPixel，將圖片轉換為灰階。

using System;
using System.Diagnostics;
using System.Drawing;
using System.IO;
using System.Windows.Forms;

namespace Test
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string Photo_route = Path.Combine(System.AppDomain.CurrentDomain.SetupInformation.ApplicationBase, "Tulips.jpg");
            Bitmap b = (Bitmap)Bitmap.FromFile(Photo_route);
            pictureBox2.Image = (Bitmap)Bitmap.FromFile(Photo_route);
            Stopwatch sw = new Stopwatch();
            sw.Start();
            //對每個pixel取出處理
            for (int y = 0; y < b.Height; y++)
            {
                for (int x = 0; x < b.Width; x++)
                {
                    Color color = b.GetPixel(x, y);
                    int avg = (color.R + color.G + color.B) / 3; //RGB同除3就會變成灰階
                    b.SetPixel(x, y, Color.FromArgb(avg, avg, avg));
                }
            }
            sw.Stop();
            pictureBox1.Image = b;
            MessageBox.Show(sw.ElapsedMilliseconds.ToString() + " /ms");
        }
    }
}

※因為將每個 pixel 取出後再寫入，所以效能會較差！