There does appear to be two distinct colors of grey, I'm not taking sides here just an observation of what I've seen at Mildenhall, the weather also has an effect on the hue, clear skies produce a lighter color but there is a difference, the lighter seems more common from my observations. Aerial shots are hard to get accurate tones from due to gamma changes from the exposure, but these two do show a marked difference.
I personally think the current Milviz one is a little too dark, but accept that could be down to users screen set ups etc and screen shots which can be misleading. This is one of the problems of producing a product suitable for a vast array of platforms, for some it'll be too dark, for others just right. Note also that aircraft, especially matt finished ones, fade very quickly, weather changes paint like nothing else can.