In mission editor, press F6 or click on the Marker section.
Then when you double click on a part of the map, a dialog will appear. In that dialog, I think there is a 2 selection radio button that says "Icon" or "Area" (or something like that", and it's selected to "Icon" by default. Change it to "Area" and you'll have different colour and shading that you can put on a map.
Hope that helped
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