segmentation with RGB value

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i make a code for segmentation image
my goal is my image(leion of skin cancer) segmented with normal skin and lesion skin
this my algorithm :
1. get value pixel (0,0) for seed pixel
2. compare value seed pixel with one neighbor pixel
3. if value of no.3 less than treshold (T), go to next pixel and go to no.2
4. if value of no.3 more than treshold (T), change pixel to white(also for next 10 pixel), and get new seed value pixel.
(my image have greyscale before)

private void button4_Click(object sender, EventArgs e)
        {
            // GDI+ still lies to us - the return format is BGR, NOT RGB.
            BitmapData bmData = RImage.LockBits(new Rectangle(0, 0, RImage.Width, RImage.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

            int stride = bmData.Stride;
            System.IntPtr Scan0 = bmData.Scan0;

            unsafe
            {
                byte* p = (byte*)(void*)Scan0;

                int nOffset = stride - RImage.Width * 3;

                for (int y = 0; y < RImage.Height; ++y)
                {                   
                    for (int x = 0; x < RImage.Width; ++x)
                    {  
                        if (x == 0)
                        {                            
                            //RImage.GetPixel(x, y);
                            seedR = p[x];
                            seedG = p[x+1];
                            seedB = p[x+2];
                        }
                        
                        if ((seedR - p[x] >= tred) || (p[x] - seedR >= tred))
                        {
                            //taruh value p dlu disini
                            for (int i=1; i <= 5; ++i)
                            {
                                p[x] = p[x + 1] = p[x + 2] = 0;
                                x++;
                            }

                            //RImage.GetPixel(x, y);
                            seedR = p[x];
                            seedG = p[x + 1];
                            seedB = p[x + 2];
                        }
                        p += 3;
                    }
                    p += nOffset;
                }
            }

            RImage.UnlockBits(bmData);
        }

my problem is my image become white in 1/3 of image
what wrong this??