[ferret_users] convert color figure to black figure

[yangxing zheng <yangxingz@xxxxxxxxx>]

Hi,

   Usually we make shading contour using fill, or shade in ferret, and the picture is colorful. Now I am going to use black/white instead. How can I convert the color figure to black/white one within ferret or with unit command?

   Thanks!

   Yangxing