How to change colors to black-white into the top left triangle of the screen?

    public Bitmap MakeScreenshot()
    {
        Bitmap Bitmap = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
        Graphics Graphics = Graphics.FromImage(Bitmap);
        Graphics.CopyFromScreen(0, 0, 0, 0, Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);

        return Bitmap;
    }

    //Reduce Screenshot Method
    public static void ReduceScreenshot(string fileName)
    {
        var bmpSS = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
        var gfxSS = Graphics.FromImage(bmpSS);
        gfxSS.CopyFromScreen(Screen.PrimaryScreen.Bounds.X, Screen.PrimaryScreen.Bounds.Y, 0, 0, Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);

        ColorMatrix colorMatrix = new ColorMatrix(
           new float[][]
           {
               new float[] { 1.5f, 1.5f, 1.5f, 0, 0},
               new float[] { 1.5f,1.5f, 1.5f, 0, 0},
               new float[] {1.5f, 1.5f, 1.5f, 0, 0},
               new float[] {0, 0, 0, 1, 0},
               new float[] {-1, -1, -1, 0, 1}
           });

        ImageAttributes attributes = new ImageAttributes();
        attributes.SetColorMatrix(colorMatrix);
        Rectangle abc= new Rectangle(-783, -383, bmpSS.Width, bmpSS.Height);
        gfxSS.DrawImage(bmpSS, abc, -783, -383, bmpSS.Width, bmpSS.Height, GraphicsUnit.Pixel, attributes);

        bmpSS.Save("ScreenshotGray.png", ImageFormat.Png);
    }

}