Darren,
Looking at my F-117 references, I do see quite a few cases of certain panels being a signifigantly different shade of dark grey. Especially the areas right above the exhausts and immediately behind the cockpit on top of the spine. This photo shows what I mean:
This next photo shows a light-greyish color in and around the panel lines:
If it were up to me, I would pick a couple of different shades of dark greay, then mask off those areas that appear different and spray, then mask off a couple of others, then spray. After that, I would use a HEAVILY thinned dark grey and overspray the entire aircraft to blend all of the colors together to avoid the "patchwork" look. Then, I would mix up a medium grey wash for the panel lines, and add some white and also some burnt umber pastels for the exhaust area.