从位图中将所有像素的颜色统计出来，然后根据各种颜色出现的次数做个排序。

        class PixelColor : IComparable<PixelColor>
        {
            private System.Drawing.Color _Color;
            private int _Count;

            public System.Drawing.Color Color
            {
                get
                {
                    return _Color;
                }

                set
                {
                    _Color = value;
                }
            }

            public int Count
            {
                get
                {
                    return _Count;
                }

                set
                {
                    _Count = value;
                }
            }

            public PixelColor(System.Drawing.Color color)
            {
                Color = color;
            }

            static public PixelColor operator ++(PixelColor pixelColor)
            {
                if (pixelColor != null)
                {
                    pixelColor.Count++;
                    return pixelColor;
                }
                else
                {
                    return null;
                }
            }

            public override string ToString()
            {
                return string.Format("Color={0} Count={1}", Color, Count);
            }


            #region IComparable<PixelColor> Members

            public int CompareTo(PixelColor other)
            {
                return other.Count.CompareTo(this.Count);
            }

            #endregion
        }

        static private List<PixelColor> GetPixelColorFromImageFile(string fileName)
        {
            System.Drawing.Image image = System.Drawing.Image.FromFile(fileName);
            System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(image);

            Dictionary<System.Drawing.Color, PixelColor> _ColorDict = new Dictionary<System.Drawing.Color, PixelColor>();
           
            for(int x = 0; x < bitmap.Width; x++)
            {
                for(int y = 0; y < bitmap.Height; y++)
                {
                    System.Drawing.Color color = bitmap.GetPixel(x, y);

                    if (!_ColorDict.ContainsKey(color))
                    {
                        _ColorDict.Add(color, new PixelColor(color));
                    }

                    _ColorDict[color]++;
                }
            }

            List<PixelColor> pixelColorList = new List<PixelColor>();
           
            foreach (PixelColor pixelColor in _ColorDict.Values)
            {
                pixelColorList.Add(pixelColor);
            }

            pixelColorList.Sort();

            return pixelColorList;
        }

调用代码

            List<PixelColor> pixelColorList = GetPixelColorFromImageFile(@"d:\test\logo.gif");

           
            for(int i = 0 ; i < 10; i++)
            {
                Console.WriteLine(pixelColorList[i]);
            }

执行结果：

Color=Color [A=255, R=255, G=255, B=255] Count=8384
Color=Color [A=255, R=227, G=62, B=6] Count=964
Color=Color [A=255, R=234, G=110, B=68] Count=348
Color=Color [A=255, R=153, G=153, B=153] Count=283
Color=Color [A=255, R=253, G=250, B=250] Count=270
Color=Color [A=255, R=248, G=207, B=193] Count=204
Color=Color [A=255, R=241, G=158, B=130] Count=204
Color=Color [A=255, R=254, G=255, B=249] Count=180
Color=Color [A=255, R=57, G=116, B=196] Count=152
Color=Color [A=255, R=58, G=114, B=189] Count=104