此方法參照msdn上的畫8位灰階gif圖片的方法。
函數1,2為畫自定義彩色圖片方法。
函數3（GetColorPalette）和函數4（SaveGIFWithNewGrayscale）為msdn原方法。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Drawing;
using System.Drawing.Imaging;

namespace Em_Picture
{
    class Class2
    {
        public static List<Color> cl;
        //函數1：
        static void DrawPicture()
        {
            Image image = Image.FromFile("加載你的圖片：比如一個總共11色的股票行情走勢圖，其白色為背景色");
            //獲取11色相對應的調色板
            ColorPalette pal = GetColorPalette(11);
            //列出圖片中所包含的顏色
            cl = new List<Color>();
            cl.Add(Color.FromArgb(255, 0,   0,   0  ));
            cl.Add(Color.FromArgb(255, 255, 0,   0  ));
            cl.Add(Color.FromArgb(255, 51,  51,  102));
            cl.Add(Color.FromArgb(255, 102, 102, 102));
            cl.Add(Color.FromArgb(255, 0,   128, 0  ));
            cl.Add(Color.FromArgb(255, 128, 128, 128));
            cl.Add(Color.FromArgb(255, 206, 158, 0  ));
            cl.Add(Color.FromArgb(255, 200, 200, 200));
            cl.Add(Color.FromArgb(255, 206, 206, 206));
            cl.Add(Color.FromArgb(255, 206, 207, 156));
            cl.Add(Color.FromArgb(255, 255, 255, 255));
            //重寫調色板前11色
            for (int i = 0; i < cl.Count; i++)
            {
                pal.Entries[i] = cl[i];
            }
            pal.Entries[10] = Color.FromArgb(0, 255, 255, 255); // 將其alpha值修改為0，可使背景通透
            //調用畫8位彩色圖函數
            SaveGIFWithNewColorTable(image, pal, "你要保存新圖片的位置");
        }
        //函數2：
        public static void SaveGIFWithNewColorTable(Image image, ColorPalette pal, string filename)
        {
            int Width = image.Width;
            int Height = image.Height;
            Bitmap bitmap = new Bitmap(Width, Height, PixelFormat.Format8bppIndexed);
            bitmap.Palette = pal;
            Bitmap BmpCopy = new Bitmap(Width, Height, PixelFormat.Format32bppArgb);
            {
                Graphics g = Graphics.FromImage(BmpCopy);
                g.PageUnit = GraphicsUnit.Pixel;
                g.DrawImage(image, 0, 0, Width, Height);
                g.Dispose();
            }
            BitmapData bitmapData;
            Rectangle rect = new Rectangle(0, 0, Width, Height);
            bitmapData = bitmap.LockBits(rect, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);
            IntPtr pixels = bitmapData.Scan0;
            BitmapData bmData = BmpCopy.LockBits(rect, ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);
            unsafe
            {
                byte* pBits;
                if (bitmapData.Stride > 0)
                    pBits = (byte*)pixels.ToPointer();
                else
                    pBits = (byte*)pixels.ToPointer() + bitmapData.Stride * (Height - 1);
                uint stride = (uint)Math.Abs(bitmapData.Stride);
                //
                byte* p = (byte*)bmData.Scan0;
                int offset = bmData.Stride - Width * 4;
                //
                for (uint row = 0; row < Height; ++row)
                {
                    for (uint col = 0; col < Width; ++col)
                    {
                        //取原圖中某個點顏色值
                        Color pixel = Color.FromArgb(p[3], p[2], p[1], p[0]);
                        p += 4;
                        //獲取新圖對應點的指針
                        byte* p8bppPixel = pBits + row * stride + col;
                        //將原圖這個點的顏色所對應的顏色索引值賦給指針
                        *p8bppPixel = (byte)cl.IndexOf(pixel);
                       
                        //===========================================================
                        //其實上面的代碼比較消耗性能，改為如下代碼較好（也可使用switch函數）
                        /*
                        byte b = 0;
                        //判斷的順序為建立顏色表的倒序
                        //因為白色為背景色，絕大多數判斷到這里就會停止
                        if (p[1] == 255) b = 10;      
                        else if (p[1] == 207) b = 9;
                        else if (p[1] == 206) b = 8;
                        else if (p[1] == 200) b = 7;
                        else if (p[1] == 158) b = 6;
                        else if (p[1] == 128 && p[2] == 128) b = 5;
                        else if (p[1] == 128 && p[2] == 0) b = 4;
                        else if (p[1] == 102) b = 3;
                        else if (p[1] == 51) b = 2;
                        else if (p[1] == 0 && p[2] == 225) b = 1;
                        else if (p[1] == 0 && p[2] == 0) b = 0;
                        p += 4;
                        byte* p8bppPixel = pBits + row * stride + col;
                        *p8bppPixel = b;
                        */
                        //===========================================================
                    } /* end loop for col */
                    p += offset;
                } /* end loop for row */
                BmpCopy.UnlockBits(bmData);
            } /* end unsafe */
            bitmap.UnlockBits(bitmapData);
            bitmap.Save(filename, ImageFormat.Gif);
            BmpCopy.Dispose();
            bitmap.Dispose();
        }

        //函數3：獲取顏色個數相對應的調色板 
        public static ColorPalette GetColorPalette(uint nColors)
        {
            // Assume monoChrome image.
            PixelFormat bitscolordepth = PixelFormat.Format1bppIndexed;
            ColorPalette palette;       // The Palette we are stealing
            Bitmap bitmap;              // The source of the stolen palette
            // Determine number of colors.
            if (nColors > 2)
                bitscolordepth = PixelFormat.Format4bppIndexed;
            if (nColors > 16)
                bitscolordepth = PixelFormat.Format8bppIndexed;
            // Make a new Bitmap object to get its Palette.
            bitmap = new Bitmap(1, 1, bitscolordepth);
            palette = bitmap.Palette;   // Grab the palette
            bitmap.Dispose();           // cleanup the source Bitmap
            return palette;             // Send the palette back
        }
        //函數4：畫8位灰階gif圖    
        public static void SaveGIFWithNewGrayscale(Image image, string filename, uint nColors, bool fTransparent)
        {
            if (nColors > 256)
                nColors = 256;
            if (nColors < 2)
                nColors = 2;
            int Width = image.Width;
            int Height = image.Height;
            Bitmap bitmap = new Bitmap(Width, Height, PixelFormat.Format8bppIndexed);
            ColorPalette pal = GetColorPalette(nColors);
            for (uint i = 0; i < nColors; i++)
            {
                uint Alpha = 0xFF;                              // Colors are opaque.
                uint Intensity = i * 0xFF / (nColors - 1);      // Even distribution.
                if (i == 0 && fTransparent)                     // Make this color index
                    Alpha = 0;                                  // Transparent
                pal.Entries[i] = Color.FromArgb((int)Alpha, (int)Intensity, (int)Intensity, (int)Intensity);
            }
            bitmap.Palette = pal;
            Bitmap BmpCopy = new Bitmap(Width, Height, PixelFormat.Format32bppArgb);
            {
                Graphics g = Graphics.FromImage(BmpCopy);
                g.PageUnit = GraphicsUnit.Pixel;
                g.DrawImage(image, 0, 0, Width, Height);
                g.Dispose();
            }
            BitmapData bitmapData;
            Rectangle rect = new Rectangle(0, 0, Width, Height);
            bitmapData = bitmap.LockBits(rect, ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);
            IntPtr pixels = bitmapData.Scan0;
            unsafe
            {
                byte* pBits;
                if (bitmapData.Stride > 0)
                    pBits = (byte*)pixels.ToPointer();
                else
                    pBits = (byte*)pixels.ToPointer() + bitmapData.Stride * (Height - 1);
                uint stride = (uint)Math.Abs(bitmapData.Stride);
                for (uint row = 0; row < Height; ++row)
                {
                    for (uint col = 0; col < Width; ++col)
                    {
                        Color pixel;        // The source pixel.
                        byte* p8bppPixel = pBits + row * stride + col;
                        pixel = BmpCopy.GetPixel((int)col, (int)row);
                        double luminance = (pixel.R * 0.299) + (pixel.G * 0.587) + (pixel.B * 0.114);
                        *p8bppPixel = (byte)(luminance * (nColors - 1) / 255 + 0.5);
                    } /* end loop for col */
                } /* end loop for row */
            } /* end unsafe */
            bitmap.UnlockBits(bitmapData);
            bitmap.Save(filename, ImageFormat.Gif);
            BmpCopy.Dispose();
            bitmap.Dispose();
        }
    }
}