Why does this happen to the image when i use writablebitmap is it reading it wrong? do i need to have an offset?

Question

I have been trying to load an image (which works) as a bitmapimage then convert that writeablebitmap to grey scale by averaging all of the colours together how ever when ever i do it the image has green or blue and red line across seemly dependent if the offset was even or odd. I manged to change that so its white lines with weird messed up RGB across sections of the image. I dont know if its my code a common problem with writeablebitmap.

images

this is what the image i am loading into the program looks like

this is the image after i run my program

code

void imageLoad()
        {

            BitmapImage bit = new BitmapImage();
            bit.BeginInit();

            bit.UriSource = new Uri(@"amazon.jpg", UriKind.Relative);
            bit.EndInit();


            storeinarray(bit);
        }

        void storeinarray(BitmapImage bit)
        {
            WriteableBitmap wrbit = new WriteableBitmap(bit);

            int pxWidth = wrbit.PixelWidth;
            int pxHeigth = wrbit.PixelHeight;
            int numColours = pxWidth * pxHeigth * 4;
            int[,][] colourData = new int[pxWidth, pxHeigth][];

            int stride =(bit.PixelWidth * bit.Format.BitsPerPixel+7) / 8;

            byte[] array = new byte[stride * bit.PixelHeight];


            bit.CopyPixels(array, stride, 0);


            for (int x = 0; x < pxWidth; x++)
            {
                for (int y = 0; y < pxHeigth*4; y+=4)
                {
                    colourData[x, y/4] = new int[4];
                    colourData[x, y/4][0] = array[x + y];
                    colourData[x, y/4][1] = array[x + y + 1];
                    colourData[x, y/4][2] = array[x + y + 2];
                    colourData[x, y/4][3] = array[x + y + 3];
                }
            }

            changeImage(colourData, bit, stride);
        }

        void changeImage(int[,][] colourdata, BitmapImage bit, int stride)
        {

            int avg = 0;
            WriteableBitmap writeable = new WriteableBitmap(bit);

            Int32Rect int32rect = new Int32Rect(x: 0, y: 0, width: writeable.PixelWidth, height: writeable.PixelHeight);

            byte[] dataforwr = new byte[stride * bit.PixelHeight];
            Random ran = new Random();
            for (int x = 0; x < colourdata.GetLength(0); x++)
            {
                for (int y = 0; y < colourdata.GetLength(1); y++)
                {

                    avg = ((colourdata[x, y][0] + colourdata[x, y][1] + colourdata[x, y][2]) / 3);
                    colourdata[x, y][0] = colourdata[x, y][0];//B
                    colourdata[x, y][1] = colourdata[x, y][1];//G
                    colourdata[x, y][2] = colourdata[x, y][2]; //R
                    colourdata[x, y][3] = 255;//A



                }
            }

            int xs = 0;
            int ys = 0;
            for (int i = 0; i < dataforwr.Length / 4; i += 4)
            {
                dataforwr[i] = (byte)colourdata[xs, ys][0];
                dataforwr[i + 1] = (byte)colourdata[xs, ys][1];
                dataforwr[i + 2] = (byte)colourdata[xs, ys][2];
                dataforwr[i + 3] = (byte)colourdata[xs, ys][3];

                ys++;
                if (ys + 1 == writeable.PixelHeight)
                {
                    xs++;
                    ys = 0;
                }


            }

            int buffersize = stride * writeable.PixelHeight;



            writeable.WritePixels(int32rect, dataforwr, stride, 0);



            Image iamge = new Image();
            iamge.Source = bit;

            Canvas.SetTop(iamge, 100);
            can.Children.Add(iamge);

            Image image = new Image();
            image.Source = writeable;

            // image.Stretch = Stretch.UniformToFill;

            can.Children.Add(image);


        }

I have tried a solid block of colour for each pixel and all the pixel come out the same colour but then mix where there are green line and such e.g i set a solid colour red so A=255,R=255,B=0,G=0 the back ground would be red with yellow lines where the green and red have been put together