C#中可以用ColorMatrix进行颜色变换。ColorMatrix由一个float[][]交错数组组成，5行5列。

横行表示5个颜色源：源R、源G、源B、源Alpha、源1

纵列表示5个颜色目标：目标R、目标G、目标B、目标Alpha、目标1（忽略）

灰度化公式：light = 0.299 * r + 0.587 * g + 0.114 * b

源代码如下：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Imaging;
using System.Text;
using System.Windows.Forms;

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

        Bitmap colorBmp;
        Bitmap grayBmp;

        Bitmap GetGrayScale(Bitmap src)
        {
            // 灰度化公式：
            // light = 0.299 * r + 0.587 * g + 0.114 * b
            float[][] cmatdata =
                    {
                        new float[] {0.299f, 0.299f, 0.299f, 0, 0},
                        new float[] {0.587f, 0.587f, 0.587f, 0, 0},
                        new float[] {0.114f, 0.114f, 0.114f, 0, 0},
                        new float[] {0, 0, 0, 1, 0},
                        new float[] {0, 0, 0, 0, 1},
                    };

            ColorMatrix cmat = new ColorMatrix(cmatdata);

            ImageAttributes imageattr = new ImageAttributes();

            imageattr.SetColorMatrix(cmat);

            Bitmap graybmp = new Bitmap(src.Width, src.Height);

            Graphics.FromImage(graybmp).DrawImage(
                src,
                new Rectangle(0, 0, src.Width, src.Height),
                0, 0, src.Width, src.Height,
                GraphicsUnit.Pixel,
                imageattr
                );

            return graybmp;
        }
        
        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Title = "打开图片";
            if (ofd.ShowDialog() == DialogResult.OK)
            {
                try
                {
                    colorBmp = new Bitmap(ofd.FileName);

                    grayBmp = GetGrayScale(colorBmp);

                    Invalidate();
                }
                catch (Exception err)
                {
                    MessageBox.Show(err.ToString() + "\r\n" + err.StackTrace);
                }
            }
        }

        private void Form_Paint(object sender, PaintEventArgs e)
        {
            if (grayBmp != null)
            {
                e.Graphics.DrawImage(grayBmp, 0, 0, grayBmp.Width, grayBmp.Height);
            }
        }
    }
}