Can't clear bitmap with Color.Orange

Question

I am creating icons on the fly for a set of winforms/clickonce applications. I have it all working by following http://www.vbforums.com/showthread.php?396650-Create-Valid-Icon-Files!-In-24-bit-true-color and adapting to C#.

I would like to have a different background colour for different environments, e.g. Live has a white background, QA is yellow, and DEV should be orange. All seems pretty easy and I have been able to create my temp.ico with white and yellow backgrounds using the following:

    private static void CreateTempIcon(int size)
    {
        var myBmp = new Bitmap(size, size);
        var graphics = Graphics.FromImage(myBmp);
        var bgColor = GetIconBgColor();

        graphics.Clear(bgColor);

        var myIcon = Icon.FromHandle(myBmp.GetHicon());
        var st = new FileStream(TempIcon, FileMode.Create);
        var wr = new BinaryWriter(st);
        myIcon.Save(st);
        myBmp.Dispose();
        wr.Close();
    }

    private static Color GetIconBgColor()
    {
        switch (ApplicationSettings.Branch)
        {
            case BranchType.Live:
                return Color.White;
            case BranchType.Dev:
                return Color.Orange;
            case BranchType.QA:
                return Color.Yellow;
            default:
                throw new ArgumentOutOfRangeException();
        }
    }

All of the colours show correctly, except for orange which shows as red.

I have tried various things including

1. Creating the bitmap with a pixelformat

   var myBmp = new Bitmap(size, size, PixelFormat.Format32bppArgb);
   

2. Creating the colour from argb

   return Color.FromArgb(255, 255, 165, 0);
   

and variations on using/not using alpha channels, using Color.FromColor, etc. It's like my colour palette is limited to only a few basic colours. The link above says the initial icon is created in 24 bit colour which as far as I know includes orange.

I don't know a huge amount about image formats (clearly!). Could anyone point me in the direction of what I am doing wrong?