bitmap.GetPixel(16,6) 

byte[] ARGB = BitConverter.GetBytes(this._wbmp.Pixels[y * this._wbmp.PixelWidth + x]);
Color c = Color.FromArgb(ARGB[3], ARGB[2], ARGB[1], ARGB[0]);

 Stream fileStream = _openFileDialog.File.OpenRead();

public Bitmap(Stream steam)
        {
            BitmapImage bitmap = new BitmapImage();
            bitmap.SetSource(steam);
            _wbmp = new WriteableBitmap(bitmap);
        }