Why is my bitmap palette null?

Question

I have the following code:

  List<Color> colors = new List<Color>();
  colors.Add(Colors.Red);
  colors.Add(Colors.Yellow);

  BitmapPalette palette = new BitmapPalette(colors);


  var bitmap = new WriteableBitmap( (int)width,
                                    (int)height,
                                    96,
                                    96,
                                    PixelFormats.Bgr32,
                                    palette);

The proble is that bitmap.Palette property is null. Why? How can I fix this problem?

Edit:

I set the pixel fortam as PixelFormats.Indexed8, but I am unable to change the pixel value in order to set a specific index form my palette. For example:

        int column = ...;
        int row = ...;

        // Reserve the back buffer for updates.
        bitmap.Lock();

        unsafe 
        {
            // Get a pointer to the back buffer.
            int pBackBuffer = (int)bitmap.BackBuffer;

            // Find the address of the pixel to draw.
            pBackBuffer += row * bitmap.BackBufferStride;
            pBackBuffer += column * 4;

            // Compute the pixel's color.
            int color_data = ???;

            // Assign the color data to the pixel.
            *((int*)pBackBuffer) = color_data;
        }



        // Specify the area of the bitmap that changed.
        bitmap.AddDirtyRect(new Int32Rect(column, row, 1, 1));

        // Release the back buffer and make it available for display.
        bitmap.Unlock();

What value have I to set to color_data in order to set the pixel color to the second palet color (that is, yellow in this case)?