Why does SolidBrush result in a pattern?

I am trying to dynamically generate a gif image of a given size and color in a HttpHandler. For some reason, the image is being generated with a modified color pattern and not the solid color that I was expecting SolidBrush to create . The following code is the simplest I can find, which exposes the problem.

private void GenerateSquareImage(String filename, String color, Int32 width)
{
    Bitmap img = new Bitmap(width, width);
    Graphics g = Graphics.FromImage(img);
    SolidBrush brush = new SolidBrush(ColorTranslator.FromHtml("#" + color));

    g.FillRectangle(brush, 0, 0, width, width);

    img.Save(filename, System.Drawing.Imaging.ImageFormat.Gif);
    img.Dispose();

    brush.Dispose();
    g.Dispose();
}

      

        
        
        
      

    

Here is an example of the results I get when passing "415976" as a color parameter:

imageshack http://img148.imageshack.us/img148/8936/colorcb5.png

(On the left is the HTML SPAN block with the background color set to # 415976. The block on the right is my graphic where you can see the colored colors.)

The result is the same if I use this code:

g.Clear(ColorTranslator.FromHtml("#" + color));

      

        
        
        
      

    

instead of calling g.FillRectangle in the above method.

UPDATE: By default, GDI + saves GIFs with a Web Safe Palette if any programmatic changes are made to Bitmap. This is what made me ignore the unsafe palette color. I was able to change the code given in Marco's post below ( http://msdn.microsoft.com/en-us/library/aa479306.aspx ) to change the palette on my newly created image so that my color is respected.