You need a separate image with the grey colors and you paint it instead of
the color version when you want.

Here's a simple algorithm for doing greyscale:

 static public Bitmap BitmapGrayScaleWeighted(Bitmap colorBitmap)
 {
  if (colorBitmap == null)
   throw new ArgumentNullException("colorBitmap",
"ImageTools.BitmapGrayScaleWeighted");

  Bitmap grayBitmap = new Bitmap(colorBitmap.Width, colorBitmap.Height);

  Color colorPixel;
  int newRGB;

  for (int y = 0; (y < colorBitmap.Height); y++)
   for (int x = 0; (x < colorBitmap.Width); x++)
   {
    colorPixel = colorBitmap.GetPixel(x, y);
    newRGB = (int)((0.212671 * colorPixel.R) + (0.715160 * colorPixel.G) +
(0.072169 * colorPixel.B));
    grayBitmap.SetPixel(x, y, Color.FromArgb(newRGB, newRGB, newRGB));
   }

  return grayBitmap;
 }

-peter