public static  void ChangeColour(this Bitmap bmp, byte inColourR, byte inColourG, byte inColourB, byte outColourR, byte outColourG, byte outColourB)

        {

            // Specify a pixel format.

            PixelFormat pxf = PixelFormat.Format24bppRgb;

            // Lock the bitmap's bits.

            Rectangle rect = new Rectangle(, , bmp.Width, bmp.Height);

            BitmapData bmpData =

            bmp.LockBits(rect, ImageLockMode.ReadWrite,

                         pxf);

            // Get the address of the first line.

            IntPtr ptr = bmpData.Scan0;

            // Declare an array to hold the bytes of the bitmap.

            // int numBytes = bmp.Width * bmp.Height * 3;

            int numBytes = bmpData.Stride * bmp.Height;

            byte[] rgbValues = new byte[numBytes];

            // Copy the RGB values into the array.

            Marshal.Copy(ptr, rgbValues, , numBytes);

            // Manipulate the bitmap

            for (int counter = ; counter < rgbValues.Length; counter += )

            {

                if (rgbValues[counter] != inColourR &&

                    rgbValues[counter + ] != inColourG &&

                    rgbValues[counter + ] != inColourB)

                {

                    rgbValues[counter] = outColourR;

                    rgbValues[counter + ] = outColourG;

                    rgbValues[counter + ] = outColourB;

                }

            }

            // Copy the RGB values back to the bitmap

            Marshal.Copy(rgbValues, , ptr, numBytes);

            // Unlock the bits.

            bmp.UnlockBits(bmpData);

        }

  public unsafe void ChangeColour(ref Bitmap bmp, byte inColourR, byte inColourG, byte inColourB, byte outColourR, byte outColourG, byte outColourB, int k)

        {

            // Specify a pixel format.

            PixelFormat pxf = PixelFormat.Format24bppRgb;

            // Lock the bitmap's bits.

            System.Drawing.Rectangle rect = new System.Drawing.Rectangle(, , bmp.Width, bmp.Height);

            BitmapData bmpData =

            bmp.LockBits(rect, ImageLockMode.ReadWrite,

                         pxf);

            // Get the address of the first line.

            var bp =(byte*) bmpData.Scan0.ToPointer();

            for (int i = ; i != bmpData.Height; i++)

            {

                for (int j = ; j != bmpData.Width; j++)

                {

                    //0.3R+0.59G+0.11B

                    if (bp[i * bmpData.Stride + j * ] == inColourB && bp[i * bmpData.Stride + j *  + ] == inColourG && bp[i * bmpData.Stride + j *  + ] == inColourR)

                    {

                        bp[i * bmpData.Stride + j * ] = outColourB;

                        bp[i * bmpData.Stride + j *  + ] = outColourG;

                        bp[i * bmpData.Stride + j *  + ] = outColourR;

                        //float value = 0.11F * bp[i * bmpData.Stride + j * 3] + 0.59F * bp[i * bmpData.Stride + j * 3 + 1] +

                        //              0.3F * bp[i * bmpData.Stride + j * 3 + 2];

                    }

                }

            }

            bmp.UnlockBits(bmpData);

        }