ATTN: It is very important to note that Natural Dry Processed coffees tend to have roast color variations due to the fact that in such a processing method you lose a level of separation. Sorting this coffee after a roast greatly diminishes many of the exotic characteristics of the cup. Tom wrote an article that somewhat addresses the issue here: http://www.sweetmarias.com/yirgacheffescreentest.html
In my own experiments, when I've sort a NP Ethiopia and taken out all of the lighter beans and any of the misshapen ones, I've ended up with a cup that had some earthy cocoa notes, but none of the exciting winey fruit notes and considerably less sweetness.
If you are buying Natural Dry Processed coffees, do not expect color uniformity, and do not roast it like a washed Central American coffee.
As far as grading these coffees, you really need to look at it on a curve and how it compares to similar coffees. This is a much different standard that what a Colombia is graded at, a much much different coffee and desired cup. The same is true for the classic Sumatra, which by many standards would not be considered a specialty coffee but is because of the specific character of that particular coffee and processing method (wet-hulled). It is debatable as far as what we want to consider true specialty, but if we try to look at every coffee through the same lens, than we are sure to miss some truly remarkable and unique coffees.